The civil code as the major law has much importance in the Romano-Germanic legal systems including the Iranian and French legal systems. Understanding theorical foundations of this code is, no doubt, effective in clarification of the contents and the manner of interpretation of its legal rules and its evolutions and thus clarifies its place in the legal system. The rationality is well inspired by the reason in the contents and benefits from the code to protect the contents. Therefore, the rationality of the civil code takes two dimentions of substantive and procedural aspects. However, this theory is not indisputable and has adversaries in French legal system; especially due to the role of the catholic tradition and the public custom in the development of the contents of most of its rules, though, the catholic tradition unlike the Shiite fiqh, is not divine based and it is built by the Church. In the Iranian Law, certain authors base the theorical foundations of the Iranian civil code on the French civil code putting emphasis on formal rationality of the civil code regarding it as the beginning of legal modernity in Iran. However, considering the religious basis of the Iranian legal system and the role of the divine tradition in the development of the legal rules, it would not be easy to accept this theory.