Closed loop pulsating heat pipes (CLPHPs) are complex heat transfer devices having a strong thermohydrodynamic coupling governing the thermal performance. Experimental studies on a CLPHP have focused on visualizing the start-up and operation mode of CLPHP or characterizing the heat transfer capability of CLPHP. In this paper, a visualization setup was established and a series of experimental observations based on the visualization setup were conducted to study the fundamental phenomena and thermal performance in CLPHPs within water. The influence characterization has been studied for the variation of thermal condition in the evaporator section of CLPHPs (large flow rate, little flow rate), inclination angel (30 °, 45 °, 60 °), filling ratio (approximately at 30%, 50%, 70%)of the CLPHP. The thermal performance of CLPHPs was mainly evaluated by calculating the effective conductivity which related to temperature distribution of both evaporator and condenser sections of the CLPHPs, also to the heat transfer between these two sections. The results indicate a strong influence of the filling ratio and the gravity on the CLPHP thermal performance. The filling ratio also affects normal startup and stable operation of the CLPHP.Copyright © 2009 by ASME