ZnO:Al films have been prepared on p-type Si substrates by DC reactive sputtering. Hall measurement showed that the ZnO:Al films exhibited apparent n type conductivity. I-V characteristics in darkness and under illumination have been performed. It was found that the illumination reverse current increased rapidly with the applied voltage and was saturated at -1 V, which was obviously different from that in darkness. This phenomenon resulted from the photovoltaic effect of heterojunction. Furthermore, in order to investigate the mechanism of photovoltaic conversion, photoconductivity and photovoltage response spectra have been studied. It was found that the photocurrent decreased sharply when the wavelength was 380 nm, and the direction of photovoltage response also changed at this point. It was presumed that this phenomenon has a close relationship with the energy band structure of the heterojunction.