I've done a Diploma IT after X & m doing B.Sc in IT via lateral entry & want to know if I can do MCA lateral entry after B.Sc in IT?

Since you are doing B.Sc through lateral entry system, surely you can take admission in M.Sc in IT by lateral entry system. That would not need the problem. I advise you to just check the colleges like University of Mumbai for this admission. It's a very good college for computer science.