All various functions of the International Maritime Organization (IMO) can be categorized into two types: legislation and regulation. IMO’s regulatory function includes different processes with various objectives, principles, and mechanisms. This study investigates one of the main IMO regulatory tools, namely, the Mandatory Audit Scheme of its Member States, by studying its objectives, principles, and processes. In general, this regulatory tool has evolved in the historical course of its formation, transforming from a self-reporting tool to a voluntary and then a mandatory one. The Mandatory audit process is also performed based on specific principles. Accordingly, each flag, coastal, and port state has unique responsibilities in addition to their general duties. Overall, the tool contributes to enhancing accountability of IMO Member States regarding their obligations under IMO Conventions. Moreover, it helps the governments identify problems regarding law enforcement as well as applicable maritime safety and environmental standards, including issues that have helped IMO develop its capacity to meet its objectives.