Can I use Serializable Attribute instead of DataContract Attribute in WCF ?

Yes it is, the only difference however is that using serializable you can only serialize fields and not properties.

Also serializable work on opt out model where in a field that don't want to get serialized has to mark itself as nonserialized, however DataContract works on OptIn approach where any property that wants to get serialized has to mark itself as DataMember.

Also serializable will serialize the assembly information in stream along with object state, however DC only writes state to stream(XML) which is what is required in SOA.

Here is a comparison:-
http://social.msdn.microsoft.com/Forums/en-US/wcf/thread/be796bd0-2502-4bd3-96d9-60bf7d68930d/
http://msdn.microsoft.com/en-us/library/ms730035.aspx

Thanks a lot .
As I understood correctly  serializable and datacontract attributes provide different serialization engines behind the scenes. If some classes  are serializable by default    and do not need 'serializable' attribute  to be serialized   like Dataset ?
Should I provide DataContract  attribute for DataSet?.
How WCF works with ADO.NET classes ?What kind of serialization it use?
And  could you  please tell  me what does it mean opt out and opt in model?