>Thank you for reply me. But i don't know what you mean? If i change conn.BeginTransaction is ok no error ?

There are two enums named IsolationLevel:

System.Data.IsolationLevel in System.Data.dll
System.Transaction.IsolationLevel in System.Transaction.dll

The one required as a parameter for SQLConnection.BeginTransaction is the first. It occurred to me that you might be inadvertently using the second....