With no doubt, the element by which we are permitted to admire or blame one for his good or bad actions is FREEdom. But what the essence of FREEdom is and how human being can get it, in a world governed by causal determinism, are two questions which most moral philosophers in west have discussed. In this regard some of them believe that compatibility between FREEdom and causal determinism is impossible, so they have denied one of them in the favor of the other and have established incompatibilism. In contrary, some others, from modern era up now, have tried to make them compatible, in order to revival moral responsibility and prevent from falling in randomness. This view has been called compatibilism. This article studies compatibilism from the viewpoints of philosophers’ such as Hobbes, Hume, Ayer, Frankfurt, Watson, Wolf and Fischer. Finally this paper it cleared that despite the rightness of compatibilism, because of their unsuccessfulness in proposing a true explanation of human FREEdom and moral responsibility, compatibilists have failed to show that this view is reasonable and justified.