Large quantities of CdS nanowires were successfully synthesized through a one-step poly (ethylene glycol)-assisted solvothermal route. The nanowires obtained were characterized by XRD, SEM, TEM, HRTEM, UV-vis and PL. The single crystalline nanowire is uniform in morphology with a [001] growth direction. Optical properties measurement reveals the quantum confinement effect of the CdS nanowires. Based on the comparative experimental results, a possible growth mechanism of CdS nanowires was proposed that the formation of CdS nanowires was controlled by the number of CdS nucleus at the initial reaction stage. The applicability of the growth mechanism for CdS nanowires was further verified by substituting poly (vinyl alcohol) for poly (ethylene glycol).