Based on Computational Fluid Dynamics(CFD) threory, a numerical simulation of free roll decay motion of a barge section without forward speed in calm water is carried out through the secondary development of the commerical CFD software FLUENT. The roll damping coefficient and the roll natural period of barge section are obtained which agree well with the corresponding experimental ones, the numerical uncertainties are shown to be less than 3%. The proposed method is shown to effectively simulate the free roll decay motion of ship, which demonstrates the ability to assess roll motion characteristics for ships effectively and conveniently.