Using transmission electron microscopy and electron diffraction, we have investigated the microstructure of a Zr55Al10Ni5Cu30 bulk metallic glass under a uniaxial compression until a failure occurs at room temperature. It is shown that the amorphous structure has changed locally in the vicinity of the failure plane after the plastic deformation.An increase in free volume is observed within the localized areas due to the concentration of plastic flow, suggesting that the increasing of local free volume dominates the deformation mechanism of the metallic glass. The effect of free