Plastic deformation of bulk metallic glass (BMG) Zr55Al10Ni5Cu30 was investigated by nanoindentation experiments at room temperature. The characters of plastic deformation were studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the discrete shear bands are formed around the indent and the local free volume is increased in the bulk metallic glass by deformation. The mechanism governing the plastic deformation was discussed in terms of the free volume mode.