Using first-principles calculations, we systematically investigated electronic structures of ZnO nanowires of various sizes modified with different surface coverages of H, F, Cl, NH2, and NH3. We found that the 50% H(O), 50% F(Zn); 100% F; 50% H(O), 50% Cl(Zn); and 50% NH 3(Zn) passivations are energetically very favorable compared with 100% H and 100% Cl passivations. The surface chemistry involved presents a strong effect on the band structure of ZnO nanowires as significant as that of quantum confinement. The 100% F passivation of the surfaces of ZnO nanowires leads to a decrease in the band gap, whereas the 100% H, 50% H(O), 50% F(Zn); 50% H(O), 50% Cl(Zn); 100% Cl; 50% H(O), 50% NH2(Zn); and 50% NH 3(Zn) passivations increase the band gap when compared with the bare wires. Like the size effect, the surface passivation is an additional option to engineer electronic properties of ZnO nanowires.