Augustine divides voluntas into two stages, before the fall and after the fall of Adam. In the debate with Fortunatus the Manichee, Augustine argues that moral responsibility is based on the autonomy, but not on the alternatives of our free choice. That is, although human voluntas, after the fall of Adam, is plunged into the necessity of sin, Adam and all the descendents still sin by the autonomy of their own free choice, and therefore have to undertake the moral responsibility for the sin. The paper will show that "involuntary sin" is a self-contradictory concept, which doesn't elucidate the theoretical reformations in Augustine' early philosophy of voluntas.