The wave making resistance theory was applied in estimating the wave resistance of trimarans. The effects of hull arrangement, hull form, ratio of displacement of three hulls, were studied both theoretically and numerically so as to obtain a favorable wave cancellation. At Tokyo University Ship Model Basin, towing tests and wave observations were carried out, using Inuid type models ( L =2. 0 m each) having a low wavemaking level at F_n =0. 3162. Experimental results showed fairly good agreement, in every case i. e. single-hull, twin-hull and three-hull arrangement, with theoretical calculation using so-called μ-correction. From resistance tests as well as from wave observations, it was noticed that reflection and diffraction of waves by the hulls are not so remarkable as to invalidate the assumption that the wave resistance of a trimaran can be calculated from the resultant wave system obtained from the linear superposition of three wave systems. Hence it might well be said that the reliability of the estimation of trimaran wave resistance depends mainly upon those of three component hulls. In the cource of resistance analysis, it was found that the form-factor K of trimarans was smaller than that of catamarans which was markedly larger than that of single hulls, in case each hull is symmetrical to its center plane.