In this study, the effects of two main parameters such as reheating temperature and holding time on microstructural evolution of Mg-6Zn-3Sm-0.4Zr alloy were investigated by isothermal heat treatment.The results show that these two parameters have important effects on microstructural evolution of the alloy during isothermal heat treatment.However, in contrast to the latter parameter, the former parameter mainly affects liquid phase amount while the holding time primarily influences the size of solid particles.As the reheating temperature or holding time increases, the microstructural evolution can be gradually accelerated from original dendritic microstructures to spheroidal or ellipsoidal particles, meanwhile, the non-dendritic particles are obtained by suitable isothermal heat treatment.With the reheating temperature elevating from 570 to 620 ℃, the liquid phase fraction gradually increases from 9 to 65 %.Furthermore, with the holding time varying from 5 to 60 min, the liquid phase fraction gradually increases from 7 to 36 % while the average size of solid particles shows fluctuant variation.The ideally spheroidal particles with an average size of 79 μm and an ideal 1.20 shape factor are obtained through the appropriate isothermal heat treatment parameters at 600 ℃ for 30 min.