The microstructure and crystallization process of an amorphous Cu56Zr44 alloy at isothermal annealing were studied by using the X-ray diffraction, differential scanning calorimetry and transmission electron micrograph techniques. The experimental results seem to show that the short-range order of amorphous Cu56Zr44 alloy at room temperature is of hard sphere random dense packing. It is found that the crystallization products of amorphous Cu56Zr44 alloy annealed at 703K within its supercooled liquid range is mainly Cu8Zr3 phase in an annealing time of 3min, and when the time is 6min, Cu8Zr3 phase disappears completely and Cu10Zr7 phase precipitates a lot, while if the time is longer than 30min, several kinds of phases such as Cu10Zr7, CuZr2 and Cu8Zr3 will form. Based on the above the diffusion mechanism of amorphous Cu56Zr44 alloy at isothermal annealing is presented.