The effect of the filling ratio (25% <= FR <= 98%) on the heat transfer performance of a novel two-phase closed thermosyphon (TPCT) combined with a superhydrophilic (SHi) evaporator and slippery lubricant-infused porous surface (SLIPS) condenser (TPCT-SHiSL) is systematically investigated in the present study. On the evaporator side, experimental results show that a different evaporation mechanism takes place dependent on the filling ratio, which has a strong impact on the overall heat transfer performance. Film evaporation plays a dominant role at the filling ratio of 25% and at the filling ratios of 40% and 70% at low heat flux. Nonetheless, film evaporation is gradually reduced and pool boiling becomes important at the filling ratios of 40% and 70% for moderate and high heat fluxes. Finally, pool boiling plays a dominant role for the TPCT-SHiSL at the filling ratio of 98%. In the case of the condenser, the condensation heat transfer of the SLIPS is reduced with increasing filling ratio. Balancing between the thermal resistance and heat transfer capacity, the TPCT-SHiSL at the filling ratio of 40% shows the best heat transfer performance. (C) 2017 Elsevier Ltd. All rights reserved.