Equity as a principle that has Roman roots and has been developed in the English legal system, has »Infra Legem«,»Praeter legem«,»Contra legem«functions and in addition to this, in many written legal systems, including Iran, it also has a legislative function, each of which has had a deep impact on the relationships especially between the administration and the citizen. Undoubtedly, the aforementioned functions have affected the Iranian administrative law so that evidence such as laws and regulations, judicial opinions and justice-based mechanisms, including the prediction of citizens' participation in administrative decisions and the ombudsman institution, etc., can be listed in it. Therefore, in this article, the aim is to define the principle of Equity, and analyze its major functions that have been influential in Iran's administrative law. The question is, what functions can be counted for the principle of Equity in Iranian administrative law? It seems that the function of »Contra legem« cannot be inferred in Iranian administrative law due to the positivist attitude of the legal system and the insistence on the rule of law principle.