Actual phase transition processes of phase change materials (PCMs) are not fully isothermal. Furthermore, PCMs may exist in the partially-melted states during their applications. In this paper, the cooling processes of sodium acetate trihydrate (SAT), starting from three different states, namely not-melted, partially-melted, and fully-melted, were studied. It was found that the original state of SAT prior to phase transition affected its performance. When SAT was in the partially-melted state and the melting ratio was low, SAT released latent heat during the cooling process and the degree of supercooling was relatively small. When the melting ratio of SAT was high or the PCM was in its fully-melted state, the degree of supercooling was high and it did not release latent heat. The results also showed that the degree of supercooling of partially-melted SAT increased with the increase of maximum heating temperature sustained time prior to the cooling process. (C) 2014 Elsevier B.V. All rights reserved.