The start-up of a two phase cooling system is a complex transient phenomenon especially for the mechanically pumped two phase cooling loop (MPCL) which is now a promising thermal control method for efficiently extracting heat from large electronic equipments. In this paper the system design and work principle as well as the test setup of a MPCL is presented and the start-up processes of the MPCL are studied. Experiments on the start-up processes under a variety of conditions were conducted. Special attention has been paid to the startups of the system in different evaporative temperature variable mass flow or heating load and some abnormal start-Lip pre-history. The transient flow exchange between the main loop and the accumulator was observed and discussed according to the different start-up patterns, which have been identified as pre-condition pump start-up and heat load start-up. During the start-up processes the system presents good stability and each part of the system performs a reasonable temperature wave except of some superheat phenomena in the evaporator. The superheat is mainly related to the evaporative temperature and the initial liquid distribution in the evaporator. In general the lower is the evaporative temperature the higher is the superheat that occurs. It also indicates that the degree of superheat is not too high when the evaporator is flooded with liquid and will not cause damage to the source equipments. In conclusion the well start-up processes in different situations may cause some liquid superheats and evaporator temperature overshoots but they will not affect much the steady state operation of the MPCL (C) 2008 Elsevier Ltd. All rights reserved.