create an empty table with the same structure but making the field u want identity.then insert the data from old table to new. drop old table (after you're sure everything is ok) rename new one to name of old one.you'll need to set identity_insert on for target table.you should do this all in T-SQL not with the GUI.something like this:Create Table newTable ( id int identity(1, 1) not null, name varchar(32), address varchar(50));GOSET IDENTITY_INSERT newTable ONINSERT newTable (id, name, address)SELECT * FROM oldTable;SET IDENTITY_INSERT newTable OFF;GO-- if all okDROP TABLE oldTable;GOEXEC sp_rename 'newTable', 'oldTable';GO