The sawdust of Japanese cypress （Chamaecyparis obtusa） and the monoterpenes limonene, α-pinene and β-pinene, had a deodorizing effect on trimethylamine (TMA). Trimethylamine concentration decreased as the amount of sawdust increased, indicating that the sawdust produced in a sawmill is useful to deodorize fishy odors. The GC-MS analysis determined that the volatile monoterpenes, limonene, α-pinene and β-pinene, are components of the essential oils of Japanese cypress. The monoterpenes also possessed a deodorizing effect on trimethylamine, with β-pinene being the most effective. The deodorizing mechanism was investigated through NMR and electron charge calculations for the monoterpenes. The NMR results showed that every monoterpene interacted weakly with trimethylamine. The protons with the strongest downfield shift were the monoterpene double-bond protons. Calculations of the electrical charges of monoterpenes showed that the double-bond protons had a larger positive charge than those of the other protons. Consequently, the positive charge of the monoterpene attracts the negative charge of trimethylamine, resulting in changes in the electric charge and/or small conformation changes in the monoterpenes. This appears to be the deodorant mechanism of monoterpenes on trimethylamine.