Fārābī's legal theory, which is among his views in civil wisdom, consists of three things: foundation, source, and purpose. The foundations of Abu Nasr al-Fārābī's virtuous law is natural law, which is compatible with the objectives of Islamic Law. Furthermore, the source of the existing laws in the virtuous city, which is codified by the first ruler, is the divine revelation and tradition. Some divine traditions or natural laws are understood by common sense, and some others, which are hidden from the understanding of the majority, are inspired by revelation to the first ruler of virtuous city, or the first ruler understands them by using his acquired intellect. The aim of the first ruler, which is also the aim of the laws of virtuous city, is the happiness. The problem of this study is to investigate and explain these three things (foundation, source, and purpose) with descriptive-analytical method, which can be considered as the theory of virtuous law, which is the same as Fārābī's legal theory.