The Core-University Scientific Corporation Program has started between Philippines and Japan from 1998. The project of fishing craft is conducted to improve the performance of Philippine outrigger craft. Most fishing craft of the Philippines are constructed in the double outrigger form with a main hull and floats at both sides. Generally, a float and its beam are made of bamboo. In this paper, the characteristics of ship motions are experimentally investigated to clarify dangerous sea conditions for the craft in waves. A computer program has been developed to calculate ship motions of the craft. The accuracy of this computer program is checked by measurement of ship motions in waves. Furthermore, results of the calculation are in good agreement with the measured ones. From these results, the outriggers increase heaving and pitching motions in head seas at the short wave length, although significantly decrease heaving and rolling motions in beam seas.