The pulsating heat pipe (PHP) is an efficient two-phase heat transfer device. Its heat transfer performance and internal working fluid flow patterns are affected by a variety of parameters. The filling ratio (FR) is one of the most important parameters, and it is usually regarded as a temperature-independent constant during the operation of the PHP. In this study, the fact that the FR is temperature dependent is pointed out, recognizing that the specific volumes of the liquid working fluid and vapor working fluid change with temperature. FR lines on a T-v diagram are used to analyze the trend of the FR with temperature. Also the influence of the variation of the FR on the heat transfer performance is discussed with the experimental data obtained from a hydrogen-filled PHP and a helium-filled PHP. For a cryogenic PHP, the optimal FR is close to the critical FR, and different initial FRs cause the PHP to either reach a dry-out limit or reach a liquid convection limit. Moreover, the temperature dependence of the FR for room temperature fluids can be ignored. (C) 2018 Elsevier Ltd. All rights reserved.