Abstract: A numerical method is presented for predicting the motion characteristics of high-speed displacement catamarans in waves. The method is based on the high-speed slender body theory, so called 2-1/2D theory, where the free-surface condition is 3D but the control equation and body surface condition are 2D. The transient free-surface Green function is used to formulate the integral equation on the body surface. The numerical predictions are compared with experimental results of a twin-hull NPL ship, as well as theoretical results by the strip method. The results show that the present method can satisfactorily predict the motion response characteristic for high-speed displacement catamarans, but the strip theory cannot give rational result, because hydrodynamic interference phenomena would happen at some discrete frequencies when applying the strip theory to the calculation of hydrodynamic coefficients of multiple cylinders.