Abstract: In order to investigate the start-up characteristics of MPLHP(Mechanicalty Pumped Loop Heat Pipe),tests were carried out with different operation conditions. The results show that start-up can cause superheat for the evaporator.Moreover,superheat temperature has little relationship with mass flow and heat load but it depends on evaporative temperature.The less the evaporative temperature is,the higher the superheat temperature is.It was also pointed out that the system designer should pay more attention to the system when system start-up without liquid in the evaporator.Because a larger temperature impact would happen to electrical equipment than the start-up process with liquid work fluid filling with the evaporator.