The concept of law is one of the most fundamental concepts and theoretical bases of rule of law and has an important role in formulating the constitutionalism theory. With respect to the fact that no society has ever been lawless, and bearing in mind that the concept of law includes a formative process and is in “modern” and “traditional” forms and the modern concept of law is formed in “discontinuity” and/or “continuity” of the traditional formation, the present study indicates that the new concept of law is formed in continuation of the traditional concept with a change in the themes. Mustashār al-Dawlah was the first philosopher who realized a different fundament of law in two forms of “traditional” and “modern”. He understood the necessity of establishment of a new understanding of law and to take courses of action to realize the fact. This article entails that Mustashār al-Dawlah’s understanding of the concept of law was different from his antecedents’, and his monograph named Yek Kalameh (One Word) is the first one on the necessity of rule of law. Utilizing a change in the theme of concept of law, Mustashār al-Dawlah could change this concept, which rooted in the will of ruler and Sharia’s decree (as two bases and origins of traditional concept of law) into a modern concept of law which rooted in the will and intention of ordinary people in Mustashār al-Dawlah’s vision. Taking a descriptive-analytical method, this article aims to inquire how and what the bases and goals of Mustashār al-Dawlah were in making the change in concept of law. It seems that Mustashār al-Dawlah took the course of action relying on the bases and concepts such as “people’s will” and “people’s satisfaction” with the aim of establishing “justice”, “equality”, and “progress” as the important goals of rule of law and defined the modern concept of law from the heart of the traditional one.