Surface passivation is one valuable approach to tune the properties of nanomaterials. The piezo- electric properties of hexagonal  ZnO nanowires with four kinds of surface passivations were investigated using the rst-principles calculations. It is found that in the 50% H(O), 50% Cl(Zn); 50% H(O), 50% F(Zn) passivations, the volume and surface eects both enhance the piezoelectric coecient. This diers from the unpassivated cases where the surface eect was the sole source of piezoelectric enhancement. In the 100% H; 100% Cl passivations, the piezoelectric enhancement is not possible since the surface eect is screened by surface charge with weak polarization. The study reveals that the competition between the volume eect and surface eect in uences the iden- tication of the diameter-dependence phenomenon of piezoelectric coecients for ZnO nanowires in experiments. Moreover, the results suggest that one eective means of improving piezoelectricity of ZnO nanowires is shrinking axial lattice or increasing surface polarization through passivation.