The effect of Nb content on the microstructure and mechanical properties of Zr-based bulk metallic glass (BMG) were investigated. The addition of Nb led to the formation of the Zr-based metallic glass composites with a ductile dentritic phase by in situ precipitation. The presence of the in situ precipitated phase enhanced significantly the plasticity of the composite under uniaxial compressive test. The interactions between the precipitated phase and the shear band affect the deformation mechanism and fracture mode of the BMG by enhancing the affecting level of the normal stress on the shear surface, and the constant alpha in the Mohr-Coulomb criterion can reflect the extent of the interactions among particles and the amorphous matrix.