I am using MongoDB.I am unable to write Linq to Sql query.
How to write retrieve table from MongoDB.
I like what Christian says, but I think it is clear that the NORM project is dead at this point (though the source is out there freely so that could always change). For a nice LINQ interface over Mongo take a look at Fluent Mongo. Thus far I have found it to be a good experience. Interestingly the CodeProject article that gideon mentions is by the same fellow behind the Fluent Mongo lib but I can not find it mentioned in the article.
I don't know much too about mongoDb, but I do know the Database is under the NoSQL category and linq-to-sql doesn't make much sense.
http://en.wikipedia.org/wiki/NoSQL#Document_store
Scan through these posts: https://stackoverflow.com/questions/tagged/mongodb+linq
Linq is supported, see these articles:
http://wekeroad.com/2010/03/04/using-mongo-with-linq/
http://www.codeproject.com/KB/database/MongoDBCS.aspx
You cannot use LINQ-to-SQL with MongoDB on the backend, since Mongo does not use/accept/understand SQL. You can however use LINQ, if for instance you use NORM library for MongoDB.
If you are specifically after LINQ, but just a nice fluent query interface the official C# MongoDB driver from 10Gen has really nice and accesible query interface.