A bulk amorphous alloy, Zr54Cu36Al4Be6, was synthesized. This alloy shows a yield strength of ~1.8 GPa and a large plastic strain of ~20.5%, which have hardly been observed in monolithic bulk amorphous alloys. Microstructures of this alloy were observed to investigate the origin of the extraordinary plasticity of the alloy. The alloy exhibited a unique microstructure characterized by atomistic-scale phase separation, which probably stems from the large difference in mixing enthalpy between the binary pairs of Zr-Be and Cu-Be. This paper discusses the effect of atomistic-scale phase separation on the enhanced plasticity of amorphous alloys.