I have the table structure like this

AccountID Code Description AccountTypeID IsActive CloseToAccountID UseCurrentFXRate

where AccountId is primary key and CloseToAccountID is foreign key withreference to AccontID column of the same table

I have the mapping file like this

<property column="Code" type="String" name="Code" not-null="true" length="50" />
<property column="Description" type="String" name="Description" length="100" />
<many-to-one name="AccountTypeid" column="AccountTypeID" class="Model.CaxAccountType,Model" />
<property column="IsActive" type="Byte" name="IsActive" not-null="true" />
<many-to-one name="CloseToAccountid" column="CloseToAccountID" class="Model.CaxAccount,Model" />
<property column="UseCurrentFXRate" type="Boolean" name="UseCurrentfxRate" not-null="true" />

And my class defination is like this

public CaxAccount()
{
_accountid = 0;
_code = String.Empty;
_description = String.Empty;
_accounttypeid = new CaxAccountType() _isactive = new byte();
_closetoaccountid = new CaxAccount();
_usecurrentfxrate = false;
}

When i m using this for ex. using NUnit for Test purpose, it gives circular reference at _closetoaccountid = new CaxAccount();
can anyone tell what is the alternative for this kind of situations

Your mapping looks ok. But why are you instantiating CaxAccount for initializing _closetoaccountid. This will lead to a stackoverflow because every CaxAccount you create it will create another CaxAccount to initialize _closetoaccountid.

I think you should just let it initialize to null and have a setter method to set the property. It does not make much sense to initialize _closetoaccountid to a new CaxAccount when you are creating a new CaxAccount.
Hope this helps