The decomposition as well as its effects on the crystallization of the bulk metallic glass (MG) Zr41Ti14Cu12.5Ni10Be22.5 has been investigated using transmission electron microscopy (TEM). It is found that the decomposition destabilizes the MG and makes the MG thermally less stable with respect to crystallization. The effects of the phase separation on the subsequent crystallization are discussed based on the microstructural characteristics of the MG.