This paper rports the results of model test concerned with wave force on a hull of a box type working craft which is on the sea. The model test was carried out undere two conditions, the hull fixed and the hull unfixed to spuds (in later case, the hull can freely make rolling, pitching and heaving).
　The results of experiment are as follows.
　1. The differences in wave forces between two cases, a hull fixed and unfixed to spuds, are not recoginzed, and in both case, maximum wave force on a hull occurs when the ratio of wave length and ship length is apploximately 0.5.
　2. Max. wave force (Fmax), in the case of ship length/wave length ≒1/2, is expressed as follow.
　Fmax=Cw・Pmax
where Pmax=w0・B・d・H(max. supposed static wave pressure)
　w0:weight per Unite volume of water
　B:The width of craft
　d:The draught of craft
　H:Wave height
Cw is wave force coefficiency which takes the value of 1.0～1.3, but the langer the draught, the smaller value it takes.