We report first principles density functional calculations of the electronic structures and energy band-gaps (Delta E-g) in PbS nanowires (NW). The Delta E-g is tuned by varying the diameter of the NW -revealing the role of quantum confinement. Compressive radial strain (CS) on the NW is shown to be responsible for a semiconducting to metallic phase transition. When compared with the valence band, the conduction band (CB) of the NW, which has a significant contribution from the excited 3d-orbital of S, is found to be more sensitive to the CS with the CB minimum shifting towards and eventually crossing the Fermi energy as CS increases. The origin of the observed phase transition in a recent experiment is attributed to the CS on the PbS nanowire. (c) 2009 Elsevier B.V. All rights reserved.