In this study, we have investigated the influence of surface passivation on the electronic structures of silicon nanowires (SiNWs) by using density functional theory within the generalized gradient approximation. The band gaps of the CH3- and Cl-passivated SiNWs are remarkably smaller than those of the similar-sized H-passivated SiNWs. The difference in band gap is attributed to the different surface states induced by the passivating radicals. The calculated adsorption spectra of SiNWs are sensitive to the passivation way. Our results also show that wire orientation only has slight influence on the relative thermal stability of SiNWs.