A SWATH (small waterplane area twin hull) installed with stabilizing fins rolling freely in calm water was investigated by both numerical approach and experimental method. Uncertainty analysis was made to verify and validate the numerical method. Both linear roll damping and nonlinear roll damping were obtained and the contributions of the frictional damping and the stabilizing fins to the roll restoring moment were analyzed. The results illustrate that: the numerical free roll decay curve agrees well with the experiment; under the assumption of linear roll mode, the linear roll damping coefficient sees an upward trend with the increase of initial heel angle; the computed free roll decay curves using nonlinear damping coefficients agree well with the experimental results; the frictional damping and the stabilizing fins make minimal contributions to the roll restoring moment for the free roll decay motion in still water.