In this paper the forward velocity dependence of the roll damping moment on a ship is treated both experimentally and theoretically. Forced rolling tests using polynomial hull form models were conducted to measure the roll damping moment in forward motion. Influences of forward velocity, rolling frequency and principal dimensions are investigated. A theoretical prediction method, which is based upon the thin ship theory and modified to include effects of the ship width approximately, is presented. Calculated values agree well with the experiments through the velocity range in lower frequencies in spite of poor agreements in higher frequency range.