Abstract: Based on procedures for uncertainty analysis in CFD recommended by ITTC, the results of Small Waterplane Area Twin Hull (SWATH) ship longitudinal motions in regular head waves solved by RANS code are verified and validated. The verification both for grid convergence and time step convergence is investigated respectively. Finding shows that the numerical solving model is more sensitive to time step size than that of grid. According to the experimental data, numerical results are validated mostly. And then the levels of verification and validation for numerical results of SWATH ship longitudinal motions in regular head waves are established. Further, SWATH ship motions in conditions of two different wave heights as well as various wavelengths are carried out. The good agreement with experimental data showed by numerical results further proved that the proposed approach is effective and reliable to solve issues of SWATH ship motion in waves.