A micro-downsized radial-flow fan (Sirocco Fan) was designed for the local cooling of electronic devices and its performance was tested in order to analyze the scale effect. The outer diameter of the rotor is 8mm, and the blade chord length, the blade width and the blade thickness were about 2mm, 8mm and 0.2mm, respectively. The variable parameters to examine the scale effect were the diameter ... [Show full abstract] ratio D_1/D_2 of the rotor inlet to the outlet (0.4~0.75), the number of blades (8~20), scroll angles (6^o~12^o) and the number of rotor revolution (3750~8500rpm). It was found that the optimun. D_1 / D_2 was about 25% smaller than that of a non-downsized Sirocco Fan and the flow rate was considerably affected by both the blade thickness and the number of blades.