Temperature and humidity are the vital environmental factors influencing the distribution and abundance of ectothermal organisms. Also, these variables often determine the efficacy of biological control of spider mites using phytoseiid mites N. californicus, N. womersleyi and P. persimilis are promising biocontrol agents of spider mites. The combined effects of temperatures (15,20,25,30 & 35oC) and relative humidity (33,51,75 & 95%) on egg hatch, incubation period from single egg and cannibalism from five eggs experiment were tested for three phytoseiid mites. Photoperiod was set to 16L:8D and neither water nor diet was provided. The result showed that higher temperatures (30&35oC) and lower relative humidity (33&51%) caused increased egg mortality by shrinking and desiccation. The egg hatch of N. californicus and P. persimilis were higher than N. womersleyi in all conditions with the exception of 35oC and 95% RH treatment where P. persimilis did not hatch at all. The incubation periods of egg decreased as the humidity level increased even under the same temperature. The cannibalism, habit of feeding the same species was found from protonymphal and deutonymphal stages under higher humidity levels (75&95%) in the three phytoseiid species. This tendency was higher in N. californicus and N. womersleyi than in P. persimilis. The results partially explain the lower predator performance during hot summer by P. persimilis and the possibility to sustain the predator population when the prey population is low.