The effect of annealing temperature on the microstructure evolution of Zr₄₈Cu₃₆Ag₈Al₈ bulk metallic glass was investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Results show that the as-cast Zr₄₈Cu₃₆Ag₈Al₈ alloy by rapid solidi?cation exhibits a typical characteristic of the amorphous structure. After annealed at 703K / 20min, a homogeneous amorphous matrix was separated into two different glassy phases, namely, phase separation occurred. Because phase separation structure competed with amorphous matrix during isothermal annealing, this structure easily decomposed and transformed into the crystalline phase AlZr₂ and AlAg₃with annealing temperature increasing. The microstructure of Zr₄₈Cu₃₆Ag₈Al₈ bulk metallic glass undergoes the local structure difference, phase separation and nano-crystallization during heat treatment in the supercooled liquid region, which implies that the microstructure of Zr₄₈Cu₃₆Ag₈Al₈ bulk metallic glass is sensitive to the annealing temperature. In addition, the formation of phase separation can accelerate the formation of nano-crystals.