Many forest pests present a problem during wood quarantine. The effectiveness of phosphine (PH3), which is an altered form of MB, was examined in Monochamus saltuaris and M. alternatus. After 24 h, the larvae and the adults of both insects were susceptible to PH3 at 3 mg/L at 20°C. However, the larvae of both pests were rather than dead, seem to have been quiescent condition. PH3 showed increasing insecticidal activity in a time-dependent manner in the larva stage of both pests. We investigated the fumigants at all stages of M. saltuaris and M. alternatus, as well as the synergistic effects of PH3 in controlled atmospheres of 50% and 80% oxygen. The atmospheric oxidation of PH3 fumigation slightly increased the toxicity of the fumigant to larva stages in 125 L container at 4 mg/L. These results indicate that highly concentrated atmospheric oxidation of PH3 could be useful as a fumigant agent against M. saltuaris and M. alternatus.