i am completed diploma civil engineering and i can join after diploma in degree b.com
Why not? The only criteria for joining a graduation program like B.Com is that you must have completed 10+2 (maths is absolutely necessary, although I have seen few private colleges accepting Bio students as well) with 50% marks (it may vary with different institutions and universities). In your case, you have studied Mathematics in your diploma program so admission wouldn't be the problem.
Hopefully this will help you.
Thank you.