Experimental studies were carried out to understand the heat transfer characteristics of pulsating heat pipes. Two heat pipes with different working fluids of HFC-134a and butane are evaluated. Both heat pipes are made of aluminum plate and have the same width of 50 mm and thickness of 1.9 mm. The effects of heat flux, heat pipe orientation, length of cooling section, heating surface area, cooling mode (natural air convection vs. forced liquid cooling) and working fluid are examined. High thermal performance of 0.05/spl deg/C/W was demonstrated for the heat pipe using HFC-134a.