Law and, of course, criminal law have numerous sources of inference. One of the authoritative sources in Islamic criminal law and Iranian criminal law is custom and established habit. In some legal writings, it has been considered as rules that have been extracted from social phenomena and have become legal rules without the intervention of the legislator, which is considered one of the important sources of law. In the Iranian criminal law system, the validity of custom is often considered as one of the guiding sources. Custom prevents the obsolescence and wear and tear of law and always keeps it changing and dynamic in accordance with the needs of the people, so that the law evolves in line with time and the progress of society. Regardless of the fact that, considering the principle of legality of crime and punishment, custom can only inspire the legislator in enacting criminal laws and cannot independently criminalize behaviors, after the adoption of the codified law system in Iran, custom gained importance in recognizing legal concepts and inferring criminal law rules from texts. Since the legislator's assumption is that he formulates the law in customary language, custom has an important role in this regard. Also, in recognizing parts of the three elements of a crime, it is the only source of criminal law. Considering the prominent role of custom in the legal system, one of the important goals of this research was to determine how custom is used in the interpretation of criminal texts, and during the research we came to the conclusion that custom has a special place in all crimes and custom has been repeatedly cited in the text of the law.