A series of experiments was carried out to study the flow behaviour behind a rotationally oscillating cylinder at a low Reynolds number(Re=300) placed in a recirculation water channel. A stepper motor was used to rotate the cylinder clockwise-and-counterclockwise about its longitudinal axis at selected frequencies. The particle image velocimetry(PIV) technique was used to capture the flow field behind a rotationally oscillating cylinder. Instantaneous and timeaveraged flow fields such as the vorticity contours, streamline topologies and velocity distributions were analyzed.The effects of four rotation angle and frequency ratios F r(F r=f n/f v, the ratio of the forcing frequency f n to the natural vortex shedding frequency f v) on the wake in the lee of a rotationally oscillating cylinder were also examined. The significant wake modification was observed when the cylinder undergoes clockwise-and-counterclockwise motion with amplitude of π, especially in the range of 0.6≤F r≤1.0. A series of experiments was carried out to study the flow behavior behind a rotationally oscillating cylinder at a low Reynolds number (Re = 300) placed in a recirculation water channel. A stepper motor was used to rotate the cylinder clockwise-and-counterclockwise about its longitudinal axis at selected frequencies. The particle image velocimetry (PIV) technique was used to capture the flow field behind a rotationally oscillating cylinder. rotation angle and frequency ratios F r (F r = fn / fv, the ratio of the forcing frequency fn to the natural vortex shedding frequency fv) on the wake in the lee of rotationally oscillating cylinder were also examined. The significant wake modification was was observed when the cylinder undergoes clockwise-and-counterclockwise motion with amplitude of π, especially in the range of 0.6 ≦ Fr ≦ 1.0.