As vacuum cleaners become smaller, the size of their main components must decrease correspondingly. Although compact vacuum cleaners have different flow characteristics in comparison to full-size vacuum cleaners, research on compact vacuum cleaners is limited. In this study, the fan motor unit for a compact vacuum cleaner, which is 30 % smaller than a full-sized model, was evaluated by performing computational fluid dynamics analysis and design optimization. Since the flow path through the fan motor unit is the primary aspect determining the efficiency of the fan motor unit, the shape of the flow path is optimized. Through parametric studies, the dimensions of the diffuser underneath the fan, such as its thickness, fillet radius, and number of guide vanes, were selected as design variables. The design of the experiments was used to find the optimum design of the diffuser and guide vanes to maximize the flow path efficiency of the cleaner. The optimized model improved the flow path efficiency by 4.2 % in comparison to the initial model.