Mushy zone refers to the two-phase mixed region appearing between the solid and liquid region during solid-liquid phase change. The microstructural evolution within mushy zone is tightly coupled with the fluid flow, heat transfer and phase transition, as well as the related latent heat storage performance. In this paper, the microscopic structural evolution during paraffin's melting and solidification was observed in real time using a confocal optical microscope equipped with a thermal stage. Based on image postprocessing, the evolving features of the mushy zone are quantified, including the characteristic length of the interphase liquid, solid/liquid fraction, solid/liquid formation rate, and latent heat evolution. Our results indicate that three regions, i.e. the solid region, the mushy zone and the liquid region, can be distinctly identified during both the melting and solidification processes. Meanwhile, it is found that the microstructure within mushy zone during melting evolves quite differently from that in solidification process, implying that the former is not just a simple reverse process of the latter. In addition, the mushy zone constant, coming from the enthalpy-porosity method that simulates the macroscale heat transfer of solid-liquid phase change, was evaluated according to the measured microstructure feature. These results provide physical insights into the nature of mushy zone during both the melting and solidification processes, and also offer instructive guidelines for accurate modelling of macroscale solid-liquid phase change heat transfer. (C) 2019 Published by Elsevier Ltd.