Abstract: The Macrobrachium rosenbergii were fed with feedstuffs which contain 1%, 2%, 3% Cordyceps militaris mycelia. The results show that Cordyceps militaris mycelia can increase the phenoloxidase(PO) activity in serum, the superoxide dismutase(SOD) activity, glutathione peroxidase(GSH-Px) activity and decrease malondialdehye(MDA) concentration in muscle. The infection experiments show that Cordyceps militaris mycelia can significantly decrease the death rate of Macrobrachium rosenbergii which are infected with Aeromonas hydrophila.