The characteristics of fluid forces on a rotationally oscillating circular cylinder in a uniform flow are studied experimentally at various forcing frequencies and rotational amplitudes of cylinder oscillation. The measurements are made on the pressure fields around the circular cylinder by using the velocity data measured by PIV in conjunction with the pressure Poisson equation. The present result indicates that the fluid forces are enhanced by the low-frequency oscillation and are reduced at high-frequency oscillation, which are due to the modification of the wake structure by the influence of rotational oscillation. This trend is further magnified by an increase in rotational amplitude.