The composition and the antibacterial activity of the essential oil obtained from Cryptomeria japonica D. Don on oral bacteria were studied. The chemical composition of the essential oil was analysed by GC and GC-MS. Sixty-eight compounds accounting for 95.82% of the oil were identified. The main compounds in the oil were alpha-pinene (6.07%), sabinene (8.86%), terpinen-4-ol (9.77%), alpha-terpineol (6.13%), elemol (11.17%) and 10(15)-cadinen-4-ol (7.16%). The essential oil and some of its major compounds were tested for antimicrobial activity against 15 different genera of oral bacteria. The essential oil of C. japonica exhibited considerable inhibitory effects against all bacteria tested (MICs, 0.025-0.05 mg/mL; MBCs, 0.025-0.1 mg/mL), while its major compounds demonstrated various degrees of growth inhibition.