The crystal-facet effect on the structural stability and electronic properties of wurtzite InP nanowires (NWs) with different side-facets are investigated by using first-principles calculation within density-function theory. The surface-energy calculation suggests that side-facet structures of InP NWs are unreconstructed due to the fact that the low-index {11¯00} and {112¯0} facets with paired In-P dimers satisfy the electron counting rule. The calculated formation energies indicate that the structural stability of InP NWs strongly depends on their side-facets. Among considered InP NWs with different side-facets, the {11¯00} faceted NWs present the highest stability due to the relative low surface atom ratio, which is in good agreement with experimental observations where wurtzite InP NWs prefer to be surrounded by {11¯00} facets. The size dependence of NW band gap indicates that the band gap (Eg) of uniform-sized InP NWs with different side-facets follows the trend, Eg-{112¯0} > Eg-{11¯00}-{112¯0} > Eg-{11¯00}, when NW diameter is larger than 3 nm and a reverse trend is found in the smaller sized NWs (d < 3 nm). Our result opens the possibility to engineer the band gap of wurtzite InP NWs by controlling their side-facets.