Variation of the free volume of Zr60Al15Ni25 amorphous alloy during rolling plastic deformation and its influence on the mechanical property were investigated by differential scanning calorimetry (DSC). Results show that the Vickers microhardness decreases with increment of deformation degree ε, and the maximum reduction reaches about 10% from as-cast state to ε=95%. However, in the DSC curves the released energy due to relaxation before glassy transformation increases with ε increasing. It is concluded that the free volume in the amorphous alloy increases after plastic deformation. The excess free volume in shear zone maybe coalesces into nano-sized voids as deformation ceases. The existence of those voids leads to the decrease of the viscosity, i.e., the decline of the Vickers hardness.