i have no idea at all why I can't rollback my transaction.....

Hibernate version:
2.1.7

Hibernate config:
<session-factory>

<!-- hibernate.connection.provider_class net.sf.hibernate.connection.C3P0ConnectionProvider -->
<property name="c3p0.min_size">2</property>
<property name="c3p0.max_size">10</property>
<property name="c3p0.max_statements">100</property>
<property name="c3p0.timeout">100</property>
<property name="c3p0.validate">true</property>

<property name="connection.driver_class">
com.mysql.jdbc.Driver
</property>
<property name="connection.url">
jdbc:mysql://localhost:3306/testDB?autoReconnect=true&amp;relaxAutoCommit=true&amp;useUnicode=true&amp;characterEncoding=utf-8
</property>
<property name="connection.username">***</property>
<property name="connection.password">***</property>
<property name="show_sql">true</property>
<property name="dialect">
net.sf.hibernate.dialect.MySQLDialect
</property>
<property name="transaction.factory_class">
net.sf.hibernate.transaction.JDBCTransactionFactory
</property>

<!-- Mapping files -->
<mapping resource="Member.hbm.xml" />

</session-factory>

Code between sessionFactory.openSession() and session.close():
try {
Session session = HibernateUtil.currentSession();
logger.debug("begin transaction");
Transaction tx = session.beginTransaction();
session.connection().setAutoCommit(false);

primaryKey = (Integer) session.save(member);

logger.debug("try to rollback transaction");
// cannot rollback
tx.rollback();

// tried this one but not success
//session.connection().rollback();

HibernateUtil.closeSession();
}

catch (HibernateException he) {
logger.error(he.toString());
}
catch (Exception e) {
logger.error(e);
}

Name and version of the database you are using:
MySQL 4.0.18.

Debug level Hibernate log excerpt:
2005-01-23 12:11:44,015 [INFO] Configuration doConfigure - Configured SessionFactory: null
2005-01-23 12:11:44,015 [INFO] Configuration secondPassCompile - processing one-to-many association mappings
2005-01-23 12:11:44,015 [INFO] Configuration secondPassCompile - processing one-to-one association property references
2005-01-23 12:11:44,015 [INFO] Configuration secondPassCompile - processing foreign key constraints
2005-01-23 12:11:44,125 [INFO] Dialect <init> - Using dialect: net.sf.hibernate.dialect.MySQLDialect
2005-01-23 12:11:44,171 [INFO] SettingsFactory buildSettings - Maximim outer join fetch depth: 2
2005-01-23 12:11:44,171 [INFO] SettingsFactory buildSettings - Use outer join fetching: true
2005-01-23 12:11:44,187 [INFO] C3P0ConnectionProvider configure - C3P0 using driver: com.mysql.jdbc.Driver at URL: jdbc:mysql://localhost:3306/frdscafe?autoReconnect=true&relaxAutoCommit=true&useUnicode=true&characterEncoding=utf-8
2005-01-23 12:11:44,187 [INFO] C3P0ConnectionProvider configure - Connection properties: {user=system, password=password}
2005-01-23 12:11:44,343 [INFO] TransactionFactoryFactory buildTransactionFactory - Transaction strategy: net.sf.hibernate.transaction.JDBCTransactionFactory
2005-01-23 12:11:44,359 [INFO] TransactionManagerLookupFactory getTransactionManagerLookup - No TransactionManagerLookup configured (in JTA environment, use of process level read-write cache is not recommended)
Initializing c3p0 pool... com.mchange.v2.c3p0.PoolBackedDataSource@a193fe [ connectionPoolDataSource -> com.mchange.v2.c3p0.WrapperConnectionPoolDataSource@2e6c66 [ acquireIncrement -> 1, autoCommitOnClose -> false, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, idleConnectionTestPeriod -> 0, initialPoolSize -> 2, maxIdleTime -> 100, maxPoolSize -> 10, maxStatements -> 100, minPoolSize -> 2, nestedDataSource -> com.mchange.v2.c3p0.DriverManagerDataSource@1fb2ef9 [ description -> null, driverClass -> null, factoryClassLocation -> null, jdbcUrl -> jdbc:mysql://localhost:3306/frdscafe?autoReconnect=true&relaxAutoCommit=true&useUnicode=true&characterEncoding=utf-8, properties -> {user=system, password=password} ] , propertyCycle -> 300, testConnectionOnCheckout -> true ] , factoryClassLocation -> null, numHelperThreads -> 3 ]
2005-01-23 12:11:45,140 [INFO] SettingsFactory buildSettings - Use scrollable result sets: true
2005-01-23 12:11:45,140 [INFO] SettingsFactory buildSettings - Use JDBC3 getGeneratedKeys(): true
2005-01-23 12:11:45,140 [INFO] SettingsFactory buildSettings - Optimize cache for minimal puts: false
2005-01-23 12:11:45,140 [INFO] SettingsFactory buildSettings - echoing all SQL to stdout
2005-01-23 12:11:45,156 [INFO] SettingsFactory buildSettings - Query language substitutions: {}
2005-01-23 12:11:45,156 [INFO] SettingsFactory buildSettings - cache provider: net.sf.hibernate.cache.EhCacheProvider
2005-01-23 12:11:45,171 [INFO] Configuration configureCaches - instantiating and configuring caches
2005-01-23 12:11:45,609 [INFO] SessionFactoryImpl <init> - building session factory
2005-01-23 12:11:46,703 [INFO] SessionFactoryObjectFactory addInstance - Not binding factory to JNDI, no JNDI name configured
2005-01-23 12:11:46,703 [INFO] HibernateUtil currentSession - Return current session
2005-01-23 12:11:46,875 [DEBUG] MemberDAO addMember - begin transaction
Hibernate: insert into MEMBER (password, mem_nickname) values (?, ?)
2005-01-23 12:11:47,000 [DEBUG] MemberDAO addMember - try to rollback transaction
2005-01-23 12:11:47,015 [DEBUG] TestAction execute - primaryKey: 35
2005-01-23 12:11:47,015 [DEBUG] TestAction execute - ended

Need to use InnoBD tables for transactions to work in MySQL.