This paper presents experiments and visualization on the thermal performance of phase-change heat transfer for R245fa in a novel heat sink cooled by natural convection with phase transition in the circulation loop. The influence of the filling ratio and heating power on the thermal performance of the heat sink was investigated experimentally. The experimental results indicated that the filling ratio and heating power had a significant influence on the thermal performance of the system. The system could start up successfully and operated without temperature oscillations under the given heating power range and the transient thermal response of the system under the conditions of a low filling ratio and low heating power was slower than that under other conditions. Meanwhile, the system achieved an excellent heat dissipation performance and outstanding temperature uniformity. A maximum temperature difference of 5.1 K in the evaporation substrate and 3.2 K in the condensation substrate was obtained while the heating power was 400 W and the filling ratio was 60% and the thermal resistance of 4.8 x 10(-3) KEW was obtained while the heating power was 400 W and the filling ratio was 50%. Additionally, the two-phase flow of the working fluid in the passages was observed clearly and identified easily, and the evaporation of the working fluid was more intense with the increases of heating power which can be obtained by visualization results. (C) 2018 Elsevier Ltd. All rights reserved.