When the planing craft with outboard motor is running, cavitation occurs around the surface of propeller and lower unit of outboard motor. Cavitation has been classified under several categories by the feature and cause of occurrence. Among them, cloud cavitation and root cavitation lead to erosion damage on the surface of lower unit and propeller. To prevent from poor appearance or performance deterioration of outboard motor by erosion damage, it is important problem to predict the erosion occurrence. Currently we can predict the cavitation phenomena sufficiently, but the area of cavitation does not necessarily correspond with the area of erosion.In this study, we present the new method to predict the area of erosion due to cavitation using CFD (computer fluid dynamics) analysis. In order to evaluate the accuracy of erosion occurrence simulation, the simulation results are compared against the result of a full-scale cruising test. Comparison between simulation and experiment suggests as follows; (1)Regarding the lower unit of outboard, the area of paint peeling on the surface of lower unit due to erosion gives close agreement with the simulation result. (2)Regarding the propeller, the area of erosion with simulation is observed more widely than the experimental result. This result indicates that we predict not only cavitation occurrence but also erosion damage of the lower unit qualitatively with new method we presented. However we require further study in order to predict the erosion of propeller. We, therefore, conclude that this prediction method holds great potential as an effective tool that could be used to design new lower unit and propeller series that have an improved cavitation erosion resistance.