The electronic structures and work functions of hydrogen (H−), fluorine (F−), and hydroxyl (OH−) passivated silicon nanowires (SiNWs) are evaluated by DFT calculations. We reveal that the work function of SiNW depends strongly on the nature of passivating functional groups, the percentage of passivation and the surface passivated. In particular, a trend of work functions: F-SiNW > H-SiNW > OH-SiNW, is obtained. Taking H-SiNW as the reference, the increased work function in F-SiNW is attributed to the electron withdrawing effect from highly electronegative F atom. In contrast, although O atom is also highly electronegative, for OH-SiNW, such effect is countered by the resonance effect in which electron is donated back to the SiNW surfaces, resulting in reduced work function. The extent of the increment or reduction is proportional to the percentage coverage of the passivating chemicals. Moreover, the work function changes more significantly when the di-substituted (100) surfaces are passivated than that of the mono-substituted (110) surfaces. Consequently, OH-SiNW shows conjugate-liked Si–Si bonds at both the surfaces and the core. The results indicate that the work function of SiNW can be fine tuned by using selected chemical on selected surface with known amount of coverage for customizing purpose.