Injection of conidia solution of Metarhizium anisopliae isolate CQMa102 into the hemocoel of Periplaneta americana caused significant changes of the total hemocytes of the cockroach. P. americana eliminated large amount of the spores efficiently by phagocytosis and nodulation.The experiment shows that the immune system of the cockroach was hardly injured after injected with high dose spores, and the higher dose of spores was injected,the higher mortality of the insect was. However, low dose injection strengthened the immune ability of the insect in certain degree. The saline slightly induced the immune reaction of the insect.