Resumen: Francisco de Vitoria is widely known because of his contributions in the field of political and legal philosophy. However, less attention has been paid to his natural law theory, which is normally considered to be a mere repetition of that of Aquinas. That is why has been overlooked Vitoria's doctrine of God as legislator of that natural law and his doctrine about the natural formation of moral concepts, which is expounded especially in a relection given in 1535. In this paper we expound that doctrine, we show the internal problems that it may face, and how would Vitoria probably have solved these problems, had he undertaken the effort to resolve the issue.