The three-dimensional viscous flow around a jib and main sails of a sailing yacht is simulated by using of finite-volume method and zonal boundary technique. For the generation of grid system of the complicated three-dimensional geometry of sails, an interface boundary technique is introduced at the narrow space between two sails. This simulation is applied to the sail system of a IACC class boat and studies are made on the detailed flow around jib and main sails, the influence of interaction on the performance, the effect of the wind angle and so on. It is demonstrated that this method can be used as a tool for sail design and performance prediction of sails.