Criminal liability and Justification Factors are very important subjects in the criminal law. Ignorance of law is one of these factors. It means the person does an action or omission without knowing that it is illegal and offense. We deal with the effects of the ignorance of law on criminal liability in three fields:

First, in Common Law system, consist of England, United States of America and Scandinavian countries. In this system the ancient rule “ignorance of the law is no defense” is governed. But some doctrines differ from this rule and case law. There are about 13 exceptions in regard with this rule in one states of the U.S.A.

Second we deal with the issue in the Islamic Jurisprudence (Fiqh) .We declare the reasons of opponent and proponent and the conclusion that the ignorance of the law is defense when it is not by recklessness.

In the last chapter, we study the subject in the Iranian Criminal Law. After opponent and proponent reasons, we consider the effects of the ignorance of law in crimes with certain punishment (Hoodooed) such as adultery, drunkenness and theft and offenses which its penalties are blood money, retaliation, uncertain punishment and restraining punishment.

At the end, we propose some suggestions for solving this problem that is created in our era with different regulations in regard with people who are unaware of these regulations.