Deltamethrin is a widely used type 2 pyrethroid insecticide in home and agriculture because of its high insecticidal activity, environmental stability and relatively low acute toxicity. It was found that deltamethrin alters the immune response signaling pathways, but the mechanism is still under study. Thus, this study aimed to investigate the propensity of deltamethrin to cause immunotoxicity in rats. Random division of rats into three equal groups was done, Group (I): (negative control group): allowed dirking water ad libitum; Group II (positive control group): administered orally with 1mL corn oil /rat/day for 14 weeks: and Group III (deltamethrin treated group): Each rat was given deltamethrin (5mg/kg body weight/ day) dissolved in corn oil for 14 weeks. At the end of the study, blood samples and spleen tissues were collected for estimating malondialdehyde and glutathione peroxidase activities, as well as immuno-histochmistry examination. Results indicated significant increase in blood and splenic tissue malondialdehyde and significant decrease in blood and splenic tissue glutathione peroxidase activity in deltamethrin group when compared with those of negative or positive control groups. Light microscopic examination revealed disruption in splenic histoarchitecture while immunohistochemical staining showed many positive reacted cells to active caspase 3 in deltamethrin group. In conclusion, this study demonstrated that deltamethrin may have immuntoxic effects on spleen of adult rats.