In this paper, the effects of isothermal annealing on the hardness and elastic modulus of a Cu₃₆Zr₄₈Al₈Ag₈bulk metallic glass (BMG) were investigated. It has been shown that the Vickers hardness increases from 481 Hv (as-cast state) to 518 Hv after annealed for 20 min. Nanoindentation tests also showed that the hardness and elastic modulus increase from 6.45 GPa and ㄢ13.0 GPa (as-cast state) to 7.27 GPa and ㄢ30.9 GPa after annealed for 20 min, respectively. The results indicated that the free volume of the BMG decreased and nanocrystals nucleated during annealing. The annihilation of the free volume decreased the nucleation ratio/sites of the shear bands, while the nucleation of nanocrystals inhibited the propagation of the shear bands. Both effects are responsible for the enhancement of the hardness and elastic modulus of the BMG.