Mahmood Elnasser wrote:
"The problem is I dont know 8051 assembly yet."

and later wrote:
"I've been programming with assembly for years"

I presume that means you've programmed other chips in assembler, but never the 8051 before?

That should give you a good grounding in the principles of assembler, and the need to thoroughly understand the underlying hardware architecture.

"with c all my programs work first time from simple to extremely complex."

This is a pretty good approach: get it working (or "prototyped") first in 'C', then examine areas where your 'C' can be optimised, and finally, if there's still some extra needed, optimise the really critical bits in assembler.

Of course, this doesn't negate the need for a sound design and good practice from the outset!