Abstract: The research and application of trimaran push on the development of high speed ship.Seakeeping property plays an important role in the performance of the trimaran sailing at sea,while the roll radius of inertia is a basic property that will have impacts on the rolling natural period and ship stability.Because of the trimaran＇s special structure,assessment of radius of inertia is a little bit different from single hull ship.Througgh a threebody beam model and its numeration results,this paper found the key parameters,proposed an estimated formula for the radius of inertia of trimaran,and validated the feasibility of the assessment method.