Cu2O thin films have been grown on glass substrates at room temperature by reactive magnetron sputtering. As-deposited films exhibit high electrical resistivity and low optical transmittance. To improve the film properties, post annealing treatments in air at various temperatures have been performed. Low temperature annealing (<300 degrees C) avoids the film oxidation into CuO and the films remain single-phased. In this temperature range, the annealing in air enhances the transmittance in the visible region due to the decrease of the defect scattering. Moreover, the optical band gap of Cu2O thin films is enlarged from 2.38 to 2.51 eV with increasing annealing temperature. The increase of optical band gap accompanying the reduction of Urbach energy indicates that the widening of optical band gap may result from the partial elimination of defect band tail after thermal annealing in air. Combining experimental results with recent reported calculations, the peak at about 1.7 eV in photoluminescence spectra is assigned to the recombination of first conduction band minimum to copper vacancy.