The predatory mite, Neoseiulus californicus (McGregor), is an important biological control agent of Tetranychus urticae Koch in greenhouse crops. Oulenziella bakeri is an important alternative prey for the commercial production of N. californicus. To confirm that the predatory relationship between N. californicus and O. bakeri is beneficial to the reproduction of N. californicus, the functional and numerical responses of N. californicus to O. bakeri and T. urticae were compared when exposed to six prey densities (2, 4, 8, 16, 32 and 64 individuals) under laboratory conditions. An analysis of the logistic regression showed that the adult female N. californicus displayed a type II functional response when fed on O. bakeri and T. urticae eggs and nymphs. The proportion of the consumed prey by N. californicus to the total supplied prey declined, even though the absolute number of predation events increased as the prey density increased. The Holling disc equation revealed that N. californicus had greater attack rates on the eggs (0.86-1.03) and nymphs (0.73-0.93) of T. urticae than those of O. bakeri, which were (0.55–0.73) and (0.71–0.96), respectively. N. californicus had longer handling times for the eggs (0.05–0.09) and nymphs (0.07–0.12) of O. bakeri compared with those of T. urticae, which were (0.03–0.05) and (0.04–0.07), respectively. The numbers of eggs laid by N. californicus feeding on eggs and nymphs of T. urticae were greater than those feeding on O. bakeri; but as the density increased, the difference in the oviposition rates between the predators feeding on two mites decreased. The comparison of the functional and numerical responses of the predator to O. bakeri and T. urticae revealed that the former is a good alternative food in the commercial production of N. californicus.