Every one knows that NewId() generates a guid in mssql but what is NewSequentialId() ? Well if you tried to run NewSequentialId function you will get an error. NewSequencialId is used for default value of an table. If you have a table and with a guid field put that function for default value so it gains performance and especially ordered guid for inserted records. It important that ordered records shoult be same with the ordered guid on some times. So this makes ordering property, and performance inserting and selecting on that table.