We report on the photoresponse behavior of n-ZnO/p-Si photodiodes. Semiconducting n-ZnO films have been deposited on p-Si substrates at 480degreesC using various Ar/OZ ratios, 2:1, 4:1, and 6:1, to fabricate n-ZnO/p-Si photodiodes. As a laser of 670 nm wavelength illuminated the photodiodes, a maximum responsivity of 0.286 A/W and a maximum quantum efficiency of 53% were obtained at a reverse bias of 5 V from a diode prepared with an Ar/O-2 ratio of 6:1. The response time of the photodiode was as short as 35 ns as measured using pulse modulation of the illuminating laser.