ZnO films deposited by rf magnetron sputtering with different stoichimetries were made by varing Ar/O2 flow rate during the depositions, and annealed in vacuum. The photoluminescence measurements show that the films have strong blue emission. As O2 gas flow decreases, the blue emission peak moves to long wavelength side. The blue emission may correspond to the electron transition from the bottom of the conduction band to the acceptor level composed of zinc defects.