An uninterruptible power supply (UPS) system based on supercapacitor and liquid nitrogen (LN2) hybridization is first introduced in this paper. Of the newly designed UPS, the supercapacitor reacts instantaneously once the main supply fails, and it also starts the LN2 power system to produce continuing electricity for the customer. This hybrid UPS system is of environment cleanness, long life time, easy maintenaince, etc. A 10 kW model is analyzed in this study. A two-stage nitrogen expander is designed with the rated speed of 900 rpm as the long time power generation device of the LN2 cycle. The UPS starting process calculation is carried out. The results reveal that commercial supercapacitors could fulfill this request. This UPS could be a competent choice for the UPS application. Further discussion indicates the LN2 power system could be used widely from UPS to low carbon vehicles. (C) 2014 Elsevier Ltd. All rights reserved.