A caisson breakwater is one of the typical breakwaters in Japan. Each caisson is 10～20m long in ordinary design, but longer caissons can be constructed from a technical view point. In this study, wave force on a long caisson of composite breakwater is investigated theoretically and experimentally.
　The wave force is reduced due to the phase difference in oblique waves. The reduction ratio of wave force depends on the caisson length lB, the wave direction θ, the wave length L, and the sharpness of peak wave force. the reduction coefficients can be obtained based on the wave force of Goda's formula.
　Rotational force in the horizontal plane becomes significant in oblique waves with a large incident angle. Stability of the caisson against rotation can be judged from the equations of forces and rotational moments.
　The rotational stability shoudl be considered as well as the sliding stability for long caissons. However, for the caissons with lBsinθ/L<0.5, the rotational stability can be neglected.