Bulk metallic glass composites(BMGCs)reinforced by micro-(spherical size of about 10 μm) and nano-sized(20-50 nm) Al_3Zr crystals were synthesized by copper mold suction casting method. The isothermal crystallization of Cu_(40)Zr_(44)Ag_8Al_8 BMGCs in the supercooled liquid region was studied by differential scanning calorimeter(DSC), X-ray diffractometer(XRD), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). The mechanisms for nucleation and growth of the crystalline phases precipitated in two-stage crystallization process and the effect of the isothermal treatment on the in situ crystalline phases were discussed.Results show that the BMGCs experience a primary crystallization process. Nano-sized Al_3Zr and Cu_(10)Zr_7 phases precipitate from the amorphous matrix successively. Furthermore, isothermal annealing has no effect on the preexisting particles in the as-cast sample, which creates a new method to synthesize BMGCs strengthened by duplex crystalline phases with a large size difference. And the reinforced phase could also be dominated with intention if the as-cast BMGCs preparation and isothermal treatment process can be controlled more accurately. Bulk metallic glass composites (BMGCs) reinforced by micro- (spherical size of about 10 μm) and nano-sized (20-50 nm) Al_3Zr crystals were synthesized by copper mold suction casting method. The isothermal crystallization of Cu_ (40) Zr_ 44) Ag_8Al_8 BMGCs in the supercooled liquid region was studied by differential scanning calorimeter (DSC), X-ray diffractometer (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanisms for nucleation and growth of the crystalline deposition precipitated in two-stage crystallization process and the effect of the isothermal treatment on the in situ crystalline phases were discussed. Results show that the BMGCs experience a primary crystallization process. Nano-sized Al_3Zr and Cu_ (10) Zr_7 phases precipitate from the amorphous matrix successively. is, isothermal annealing has no effect on the pre -isting particles in the as-cast sample, which creates a new method to synthesize BMGCs strengthened by duplex crystalline phases with a large size difference. And the reinforced phase could also be dominated with intention if the as-cast BMGCs preparation and isothermal treatment process can be controlled more accurately.