The differences in the interpretations of natural law, result from different understandings of "Natural" and "Law". These differences are usually coming from different grounds of metaphysics and epistemology. The intent of this article is not to debate with different philosophy systems to illustrate the accomplishments in the philosophy of social science by Thomas Aquinas, because it always turns into quarrelsome arguments.The purpose of this article is grounded on Thomas Aquinas Theory of Natural Law to demonstrate his invaluable contributions in social philosophy, through objective and systematic discussions.Beside the abstraction and the introduction: The article is composed of the following paragraphs: 1. The ground and definition of "Law" according to Thomas Aquinas. 2. Natural Law as the ultimate norm of ethics and morality. 3. The inference from the universal principle of practice to the realization of commands. 4. Thomas Aquinas Natural Law and his Philosophy of Social philosophy. 5. Conclusion.