A 3-D source method is applied for computing wave loads on the coss structure of a Semi-submerged Catamaran, SSC (or Small Waterplane-Area Twin Hull, SWATH) running in regular waves. The 3-D Green function with forward speed, which has been prohibitively time consuming to apply to a SSC problem, is used with the aid of the steepest descent method. Among wave loads, a side force, which squeezes and splits two demihulls of a SSC, is crucially important in the strength design phase because of the SSC's distinctive configuration. It is shown by both theoretical computations and model experiments that the side force drastically decreases when the SSC is running. This is explained by that hydrodynamic interactions between the two demihulls, which are due to the action of free waves from one demihull to the other, decrease in the existence of the forward speed because some of the free waves flow downstream without reaching the other demihull. Fluctuating pressure distributions on SSC surface, which contribute to wave loads, are computed and discussed. Some of the computed results are successfully compared with experimental ones.