In this work, a small amount of Nb has been added in a Zr52Cu42.5Al5.5 bulk metallic glass, and a Zr52Cu42Al5.5Nb0.5 bulk metallic glass composite with dual-amorphous and nanocrystal structures has been developed for the first time. This in situ formed bulk metallic glass composite has a larger room compressive plasticity of above 13% than that of the Zr52Cu42.5Al5.5 bulk metallic glass. The excellent plasticity of the bulk metallic glass composite is attributed to the phase-separated matrix with micro-nanocrystal and the nanocrystallization during the deforming process. This work may give a new sight into design bulk metallic glass composites and the underlying mechanism for deformation.