ABSTRACT: It was found by Chaplin and by the the wave forces acting on cylinders submerged in regular and irregular waves decrease with increasing Kc number in number region (1<Kc<3). In the present paper, the effect of the steady motion of a circular cylinder on the reduction of the wave forces is experimentally and theoretically investigated in regular waves. The experimental results show that the inertia forces acting on the cylinder are significantly affected by the constant speed, and the tendency of the inertia coefficient depends on the direction of motions. The cause of this result is investigated by simulations of the vortex shedding patterns. INTRODUCTION It was first found by Chaplin (1984a, 1984b) that the wave forces acting on a circular cylinder horizontally submerged in regular waves dramatically decrease with increasing Kc number due to VISCOUS effect in very low Kc number region (1<Kc<3). In such low Kc number region it had been believed that there is no VISCOUS effect on the inertia force in plane oscillatory flow as shown by Bearman et al (1985) and Sarpkaya (1986). The authors (1988a, 1990) investigated the phenomenon experimentally and theoretically, and pointed out that the reduction of the wave forces is caused by the circulating flow created by separated vortices from the cylinder which is in much greater scale than those in sinusoidally oscillating flow at the same Kc number. They also found that the same phenomenon appears for a square cylinder, a flat plate and a three-dimensional semisubmersible (Ikeda et al, 1988a, 1988b). In irregular waves the same reduction of the wave forces occurs (Otsuka et a1, 1989, 1990). In the design of an offshore structure the current and the drifting motion should be taken into account an estimating the hydrodynamic wave forces acting on it.