Abstract: Flow visualization was investigated for pulsating heat pipes(PHPs) of internal diameters(IDs) 2.5,2,1,0.5mm using a high speed charge coupled device(CCD).Under different heat inputs,both the alternating transformation of the quasi-static phase and oscillation phase vs.time lapse and the flow patterns were recorded.Bubbly flow,slug flow,semi-annular flow,wavy-annular flow and annular flow were observed.Results show that the ID has a significant effect on the flow patterns.As the ID decreases,the formation of the vapor slugs at the evaporation section due to the small/tiny bubbles aggregation is dominated by the single bubble growth and expansion.In addition,more elongated bubbles are observed in the evaporator section,and deformation and breakup of the vapor plugs will likely occur in the condenser section by reducing the ID of the PHP.For the micro PHP,the capillary wavy-annular flow is a fundamental flow pattern,and the transition from capillary wavy-annular to capillary slug flow occurs in the condenser section.