Abstract: Numerical investigations of flow past a partially rotating, circular cross-section, stepped cylinder in uniform flow at varying velocity ratios, diameter ratios and Reynolds Numbers are performed. The resulting flow fields are analyzed visually for the effect of rotation on the wake and numerically for the effect on lift/drag forces as well as vortex shedding rates. Simulations are run where either the large or small-diameter cylinder was rotating while the other cylinder was held stationary. Simulations were performed with end plates to minimize end effects. Results show with increasing the velocity ratio when the large cylinder rotates, vortex shedding is suppressed across the entire wake span. Comparatively, an increase in velocity ratio for the small-diameter cylinder during rotation results in increased wake activity and a larger drag force.