Ultraviolet (UV) protection of a granulovirus (GV) product using iron oxide was investigated under laboratory conditions with fluorescent lamps. The UV protective activity was evaluated by comparing the pathogenicity of a GV product to the neonates of the oriental tea tortrix, Homona magnanima, in the presence or absence of iron oxide under UV irradiation. The infection rates of GV suggested that the addition of iron oxide at 1–4 mg/ml could reduce the inactivation of GV by UV irradiation to 1/6–1/18 as compared to that without it, indicating that iron oxide could be a useful UV protectant for microbial insecticide GV products.