The legislator, in all legal systems, considers some special conditions for the validity of lawful acts. In case of non â compliance of such conditions, we will meet various sanctions in terms of intensity and weakness, including: nullity (absolute or relative), lake of force, etc. In this dissertation, the concept of nullity of contracts and unilateral legal acts, and the consequences of their nullity will be studied. Furthermore, the dissertation conducts a comparative study in order to have more consideration on subject matter and also to make use of other legal systems, laws and regulations and their legal doctrines. Then we will carry out a comparative study of the laws of Egypt and France. The reason of choosing Egypt law for a comparative study, is its similarity with Iranian legal system from the viewpoint of following codified legal system and the effects of French legal system on civil and commercial regulations, as well as the attempts of both countries in utilizing jurisprudential doctrines in their legislating process.