Graphene Oxide (GO) is a new carbon material with a two-dimensional sheet structure, which has high thermal conductivity and strong hydrophilicity. In this study, influence of GO nanofluid and filling ratio on the start-up performance of a water-based PHP was investigated. The PHP was fabricated with a long copper tube, which was bent into 3 turns, with the inner and outer diameters were 2 mm and 4 mm respectively. The nanofluid was prepared by dissolving GO nanoparticles into the deionized water, with different mass fraction range (omega = 0.02 & ndash;0.1 wt%), and filling ratios (FR = 20%, 50%, 80%). The evaporation section was electrically heated, with a heating power ranged from 10 W to 30 W, while the condensation section was cooled by the forced convective wind. Results showed that both the filling ratio and the GO concentration had great effect on the start-up performance of the PHP. When the filling ratio was at 20% and 50%, the suitable introduction of GO nanofluid could improve the start-up performance of the water-based PHP, and the best concentration range was about 0.05 & ndash;0.08 wt%, for the start-up performance optimal. Especially at the filling ratio was 50%, adding 0.05 wt% GO nanoparticles could decrease the start-up temperature and time about 34 degrees C (41%) and 796 s (58%), respectively, when compared with the deionized water. In contrast, at the filling ratio of 80%, adding GO nanofluid did not significantly improve the start-up performance, even led a deterioration of the performance at a higher concentration (omega = 0.1 wt%). When compared with the GO concentration, the filling ratio had a greater impact on the start-up of PHP. As the filling ratio increased, the minimum starting heat input increased. As for thermal performance, when heat input was 30 W, GO concentration was 0.05 wt% and filling ratio was 50%, it reached the highest thermal performance enhancement rate about 54.34%, when compared with the pure water PHP. (c) 2021 Elsevier B.V. All rights reserved. Graphene Oxide (GO) is a new carbon material with a two-dimensional sheet structure, which has high thermal conductivity and strong hydrophilicity. In this study, influence of GO nanofluid and filling ratio on the start-up performance of a water-based PHP was investigated. The PHP was fabricated with a long copper tube, which was bent into 3 turns, with the inner and outer diameters were 2 mm and 4 mm respectively. The nanofluid was prepared by dissolving GO nanoparticles into the deionized water, with different mass fraction range (? = 0.02?0.1 wt%), and filling ratios (FR = 20%, 50%, 80%). The evaporation section was electrically heated, with a heating power ranged from 10 W to 30 W, while the condensation section was cooled by the forced convective wind. Results showed that both the filling ratio and the GO concentration had great effect on the start-up performance of the PHP. When the filling ratio was at 20% and 50%, the suitable introduction of GO nanofluid could improve the start-up performance of the water-based PHP, and the best concentration range was about 0.05?0.08 wt%, for the start-up performance optimal. Especially at the filling ratio was 50%, adding 0.05 wt% GO nanoparticles could decrease the start-up temperature and time about 34 ?C (41%) and 796 s (58%), respectively, when compared with the deionized water. In contrast, at the filling ratio of 80%, adding GO nanofluid did not significantly improve the start-up performance, even led a deterioration of the performance at a higher concentration (? = 0.1 wt%). When compared with the GO concentration, the filling ratio had a greater impact on the start-up of PHP. As the filling ratio increased, the minimum starting heat input increased. As for thermal performance, when heat input was 30 W, GO concentration was 0.05 wt% and filling ratio was 50%, it reached the highest thermal performance enhancement rate about 54.34%, when compared with the pure water PHP.