N-ZnO/p-Si heterojunction photodiodes have been fabricated by sputter deposition of n-ZnO films on p-Si substrates. The substrate temperatures of 300, 400, 480 and 550degreesC were taken for the n-ZnO film deposition using an Ar/O-2, ratio of 6:1. All the diodes show typical rectifying behaviors as characterized by the current-voltage (I-V) measurement in a dark room and their photoelectric effects from the diodes have been observed under illumination using monochromatic red light with a wavelength of 670 nm. Maximum amount of photo-current or responsivity is obtained under reverse bias conditions from a n-ZnO/p-Si heterojunction when the ZnO film was deposited at 480degreesC while the ZnO films deposited at 550degreesC show the best stoichiometric and crystalline quality. Junction leakage or dark current is much higher in the diode with n-ZnO deposited at 550degreesC than in the other diodes. It is thus concluded that for a photodiode the quality of the diode junction is as important as that of the n-ZnO film deposited on p-Si.