Who is actually monkey d dragon? I just know that he is the leader of revolutionary army and also father of luffy.

no body knooooowwwwwweeesssss~~~!

If Oda were to give out all his plot details and plans at the begining of his story he'd have no fans or story, you need a untold mystery to be revealed else it's not intresting.

Monkey D. Dragon. A mysterious man with a past that we have yet to uncover. He is the son of Monkey D. Garp and the Father of Monkey D. Luffy. His lover/wife/girlfriend (Luffy's mother) has yet to be uncovered, as is his Mother. He is the leader of the revolutionary army and is known as the world's most dangerous criminal as he intends to over throw the World Govt. He has a pag-ibig for freedom and he believes in fate, both of which are strong traits of a "D.". He appears to be a very intelligent man (a trait which was Nawawala on both his father and his son). He attended the Pirate King's execution and (though he is an absent-y Father) he seemed actually happy for his son becoming a pirate and living his life the way he wanted to. It is clear that he was never a good Father to Luffy, as the boy has no recollection of ever even having a Father at all. Any other information about the man is yet to be revealed. As I have already said, he is very mysterious. I hope to see madami of him.

He is exactly what you just sinabi - Luffy's dad and leader of revolutionary army. I'm not sure what kind of answer you expected, but when it is revaled, you will find out - because Eiichiro Oda will definitely not let fans down!