Based on a modular design concept, 100 kVA uninterruptible power system using double-sided air-cooled power module has been developed. An efficient air-cooling system suitable for the double-sided cooled power module is developed, and the volume of the converter adopted developed double-sided air-cooling system becomes less than half compared to the conventional converter based on single-sided cooling system. The width of the modular unit composing the rectifier, inverter, and chopper is only 50 mm so as to stack some units side by side in a smaller space and higher maintainability is achieved. The volume as well as the footprint of the developed UPS is smaller than the conventional one by 30 %.