A novel oxidant fumigation (NOF) has been considered as alternative fumigant to replace methyl bromide that is a serious ozone depleter. Its high oxidative activity has been used as a bleaching or sanitary agent. Though some reports an insecticidal activity of NOF, its insecticidal action is yet to be understood. This study was conducted with an observation of an insecticidal activity of NOF against Plodia interpunctella, which is a stored grain insect pest. Cytotoxicity test was performed by using MTT assay, NOF gave a significant cytotoxicity on both Sf9 cells and HiFive insect cell lines. Sf9 cells were higher susceptible (IC50 = 43.2+ 3.5 ppm) to chloride dioxide than HiFive cells (IC50 = 174.6 + 5.9 ppm). To understand its cytotoxic effect on P. interpunctella, the larval hemocytes were incubated in vitro with different doses of NOF for 40 min at room temperature. In a dose-dependent manner, NOF gave a significant toxicity to the hemocytes. When NOF was injected to larvae of P. interpunctella, it significantly reduced total hemocyte counts compared to control. These results indicate that NOF has cytotoxic effect against hemocytes of P. interpunctella. This hemolytic activity of NOF can be regarded as a lethal factor to the stored grain insect pest.