Reliability-based design approaches are needed for novel ships whose design and operational experiences are few and which are subject to external loads of random nature such as wave-induced loads. This paper deals with reliability assessment of Semi-Submerged Catamaran (SSC), one of the novel ships. An SSC is modeled as a spatial framework to which side force under beam sea is applied. Structural systems reliability analysis is carried out by using the method in which probabilistic plastic collapse analysis and failure mode analysis are combined. Effects of some design alternations on systems reliability are also discussed. Main conclusions are summarized as follows; (1) Reliability assessment is necessary for a novel ship such as an SSC in order to realize a rational design of the structure. (2) Systems reliability is indispensable because component reliability can not necessarily predict structural safety of the vessel. (3) Important design parameters are clarified through parametric studies. (4) Importance factor of members under damaged conditions is proposed.