We report ferromagnetic behavior in undoped ZnO nanofibers prepared by electrospinning a solution of zinc acetate and poly vinyl alcohol followed by annealing at 550 1C for about 90 min. X-ray diffraction patterns of the heat treated as-spun composite fibers reveal the formation of ZnO nanowires in wurtzite structure with no noticeable impurity phases. ZnO nanowires annealed between 500 1C and 600 1C exhibited room temperature ferromagnetism with decreasing magnetization with increasing annealing temperature. Room temperature ferromagnetism was observed in as-spun fibers annealed in air as well as under vacuum. However, vacuum annealed nanofibers show higher magnetization as compared to air annealed fibers, which indicates that oxygen vacancy is a cause for the observed room temperature ferromagnetism in the ZnO nanofibers.