Interpretation of contracts is one of the most important and at the same time most complex issues in the field of commercial and contractual law. In commercial relations, where speed, dynamism, facilitation of transactions and mutual trust are considered its basic elements, the apparent will and limited words used in the text of the contract are not always sufficient to resolve legal disputes and accurately determine the rights and obligations of the parties. In such cases, "commercial custom" plays a key and undeniable role as a reflection of practical procedures, accepted norms and reasonable expectations of businessmen in a particular industry. The present study aims to explain and examine in detail the position, principles and function of commercial custom in the process of interpreting commercial contracts in the Iranian legal system. This study, using the descriptive-analytical research method and citing reliable library sources, case law, judicial practice, and opinions of legal doctrine, attempts to scrutinize the various dimensions of this legal institution. The findings of this study show that in Iranian case law, especially with reference to Articles 220, 224, and 225 of the Civil Code and the general principles of commercial law, custom and habit not only act as one of the main sources supplementing the provisions of the contract in cases of silence or brevity of the parties, d interpretation of contractual texts; rather, they are legally obligated to provide a dynamic, fair, and practical interpretation of business by carefully ascertaining and applying established commercial custom and practice, in order to ensure legal security in the business environment.