In the present study, the effect of graphene/water nanofluid on the thermal performance of a two-phase closed thermosiphon (TPCT) has been considered. For the synthesis of the mentioned nanofluids, graphene with a thickness of 4-20 nm and length of 5-10 mu m has been employed. Due to the natural instability of graphene in polar solvents such as water, gum arabic (GA) has been utilized as a surfactant. Then, various nanofluids at weight concentrations of 0.02-1% were prepared and thermal properties were investigated at the input power of 30-150 W. In agreement with the results, as the weight concentration increased, the overall heat transfer coefficient and thermal efficiency of the TPCT were enhanced. On the other hand, increasing the nanofluid weight concentration and input power led to lower thermal resistance of the TPCT. Interestingly, the rate of change of the temperature in the evaporator has been studied as one of the key parameters affecting the thermal resistance and overall heat transfer coefficient of the TPCT. Increasing the concentration has compounded the reduction of the average temperature of evaporation, which has confirmed the reduction in thermal resistance. Meanwhile, the overall heat transfer coefficient increased with rising concentration at the permanent input power. Also, the vacuum pressure results showed that increasing the concentration of nanofluid led to the vacuum pressure drop being intensified.