Abstract: As the CII regulations took effect in 2023, shipping companies are under increasing pressure to comply with these stringent regulations. This paper presents a fleet optimization method for CII regulatory constraints, considering the difference in main engine fuel consumption performance within the fleet. Two scenarios are optimized: "Fleet scheduling considering single ship CII rating limits" and "Fleet scheduling with no E-class ships and the least used biofuel". The results demonstrate that the proposed optimization algorithm outperforms random scheduling in maintaining fleet CII compliance and reducing biofuel usage. This study will help shipping companies meet fleet CII regulations and reduce carbon emissions, while improving their ability to respond to international green and low-carbon regulations.