Hi Patmokgoko. I'm looking at several translations. The KJV (and others that usually follow its lead) seems to be saying that the 'will of God' is what is 'good and acceptable and perfect.'--in other words, adjectives that describe the will of God. Other translations are rendering it that it is the thing 'we may prove' as the will of God that is 'good, acceptable and perfect.'
Here's an example:
Rom 12:2 And be not configuring yourselves unto this age, but be transforming yourselves by the renewing of your mind, to the end ye may be proving what is the thing willed by God - the good and acceptable and perfect.
I can't tell you which one is 'correct', but it seems to me in light of the rest of scripture that to be 'good, acceptable and perfect' is what the transforming and renewing is proving 'what is the thing willed by God'. That seems to match precisely what Paul said eariler in Romans:
Rom 8:28 And we know that all things work together for good to them that love God, to them who are the called according to his purpose.
Rom 8:29 For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren.
Rom 8:30 Moreover whom he did predestinate, them he also called: and whom he called, them he also justified: and whom he justified, them he also glorified.
and elsewhere where he talks about the will of God in predestination.
Many are called...few are chosen. The ones He's chosen are going to 'prove' the will of God, and that 'proving' is what is described here (and elsewhere) as 'good, acceptable, and perfect.'
That's what I see, anyways. I hope that doesn't cloud things.