Abstarct: According to industry demand for heat transfer in less space, research in the field of two-phase heat transfer systems have been attractive for researchers in recent years. Pulsating heat pipe (PHP) is a two-phase heat transfer device that can be opened or closed loop which be placed in subset of conventional heat pipe’s. PHP is a U-shaped tube with several rounds and consists of two parts of the evaporator and condenser which also can exist an adiabatic zone between the evaporator and condenser. The amount of heat that is transferred from the evaporator to the condenser depends on the flow regime of the fluid is existing in PHP that this regime depends on many parameters such as structure of PHP, working fluid and thermo-physical parameters related to it, heat input to the evaporator and etc. Considering importance of the flow regime in PHP in heat transfer, at this work to verify the two-phase flow regime and its impact on the performance of PHP by making two close loop pulsating heat pipe (CLPHP) of Pyrex whit internal diameter 4mm and outer diameter 6mm, experiments were done by using of working fluids of ethanol, water, copper nano-fluid and acetone at filling ratios of 40, 50 and 60 percent and in two cases of vertical and horizontal mode and the effect of working fluid and thermo-physical characteristics related to it, filling ratio, number of rounds, the gravity, the use of nanoparticles in the working fluid and heat input to the evaporator were investigated. In this research according to the visual results, in general was observed 3 types of flow regime that each of them had a different impact on heat transfer and performance of CLPHP. The use of copper nanoparticles and the gravity with creating positive changes in flow regime and therefore with increasing the heat transfer and decreasing the temperature of the evaporator causes reducing the thermal resistance of the system. In the case of other working fluids that were used with respect to filling ratio, thermo-physical characteristics, the number of rounds CLPHP and other experimental conditions, best system performance was evaluated.