Fluid-dynamic forces on a rotating circular cylinder in a uniform flow were obtained by numerically solving the vorticity transport equation. the ranges of calculation are from 20 to 100 for the Reynolds number, R_e, and from 0 to 2 for the specific rotating speed, |V₀|, the ratio of the peripheral speed of the cylinder to the uniform velocity. The results of calculation show that the temporal-mean lift coefficient is nearly independent of R_e and is proportional |V₀|. On the other hand, the temporal-mean pressure drag coefficient increases proportionately to V₀² and that increment is due to induced drag in two-dimensional flow, while the skin-friction drag is independent of V₀. The amplitude of the temporally fluctuating lift coefficient is maximum at |V₀|&elDot;0.5 which is larger when R_e is larger. The amplitude and frequency seem to be strongly affected by the width of the formation region.