This paper describes the effects of a combination of rib and pulsating flow on heat transfer enhancement in an mm-scale model that simulates the narrow flow passages in cooling devices of downsized electronic equipment. This research aims to develop a novel water cooling device that increases heat transfer performance while inhibiting pumping power. Our recent study has reported that a combination of pulsating flow and rib could enhance heat transfer performance relative to the simple duct. In the present study, to verify the optimal rib height for improving heat transfer by pulsating flow, we evaluated the relationship between heat transfer performance of pulsating flow and rib height through three-dimensional computational fluid dynamics (CFD) analysis. Cooling performance index h was calculated to evaluate the improvement of the heat transfer peformance of pulsating flow relative to a steady flow. Higher height ribs help to achieve higher heat transfer performance regardless of the flow pulsation. However, pressure drop also increases significantly, and the level of the heat transfer enhancement by the higher ribs is dependent on the time-averaged Reynolds number. By evaluating the cooling performance index, we clarified the optimum rib height that can enhance heat transfer while inhibiting the increase of the pressure drop.