The insecticidal and fumigant activities of Cinnamomum cassia (Blume) bark-derived materials against the oak nut weevil (Mechoris ursulus Roelofs) were examined using filter paper diffusion and fumigation methods and compared to those of the commercially available Cinnamomum bark-derived compounds (eugenol, salicylaldehyde, trans-cinnamic acid, and cinnamyl alcohol). The biologically active constituent of the Cinnamomum bark was characterized as trans-cinnamaldehyde by spectroscopic analysis. In a test with the filter paper diffusion method, trans-cinnamaldehyde showed 100 and 83.3% mortality at rates of 2.5 and 1.0 mg/filter paper, respectively. At 2.5 mg/paper, strong insecticidal activity was produced from eugenol (90.0% mortality) and salicylaldehyde (88. 9%), whereas trans-cinnamic acid revealed moderate activity (73.3%). At 5 mg/paper, weak insecticidal activity (50.0%) was produced from cinnamyl alcohol. In a fumigation test, the Cinnamomum bark-derived compounds were much more effective against M. ursulus larvae in closed cups than in open ones. These results indicate that the insecticidal activity of test compounds was attributable to fumigant action, although there is also significant contact toxicity. As a naturally occurring insect-control agent, the Cinnamomum bark-derived materials described could be useful as a new preventive agent against damage caused by M. ursulus.