Abstract: In the production process of high-level waste(HLW) glass, there was crystalline phase in the centre area of the HLW glass tanks, due to high temperature. Crystallization increases phase interface of HLW glass, having an effect on the physical and chemical properties of HLW glass. In order to avoid the occurrence of crystallization, it’s necessary to study the crystallization behavior of HLW glass. Crystallization behaviors of the simulated HLW glass were researched by the isothermal heat treatment in this paper. The simulated HLW glass powders were respectively heated at 500, 600, 700, 800, 900 ℃ in the range from 0.5 h to 96 h. After heat treatment, the crystallinity and crystallinity phase of samples were detected by X-ray diffraction(XRD). The results of XRD show that the maximum degree of crystallinity of the simulated HLW glass appears at about 700 ℃. After isothermal heat treatment, there are three different crystals: omphacie, cerianite and albite. Omphacite is the major crystalline phase, which has always existed in the range from 600 ℃ to 900 ℃. Cerianite always precipitates in the range from 700 ℃ to 900 ℃. Albite only precipitates at 700 ℃.