High-temperature plastic flow in pre-annealed Zr₆₅Al₁₀Ni₁₀Cu₁₅ bulk metallic glass is examined in a supercooled region, in order to clarify the effect of the change in microstructure on high-temperature plastic flow. The specimen without annealing exhibits significant necking, but the elongation of 550% is obtained in the tensile test at 673 K and in a constant true strain rate of 1×10⁻² s⁻¹. In contrast, the specimen annealed at 673 K for 1.8 ks before the tensile test exhibits uniform deformation, and large tensile ductility of 1100% is obtained. In the specimen, initial stress decreases in comparison with that of the specimen without annealing, but apparent strain hardening arise during deformation. XRD analysis reveals that no crystalline phase is detected in the specimen without annealing even after deformation. In contrast in the specimen annealed at 673 K for 1.8 ks, the peaks from icosahedral phase are observed only in gage section but not in grip section after deformation. Strain hardening due to the precipitation and growth of icosahedral phase, which is enhanced by deformation, must make a positive contribution to neck stability, and results in the large elongation in the annealed specimen.