The chemical components of the essential oil obtained from Chrysanthemum indicum L. were analyzed by GC-MS. Seventy-three compounds accounting for 96.65% of the extracted essential oil were identified. The main compounds in the oil were $alpha$-pinene (4.4%), 1,8-cineole (10.4%), $alpha$-thujone (6.05%), camphor (10.12%), terpinen-4-ol (3.4%), bornyl acetate (6.1%), borneol (3.6%), cis-chrysanthenol (3.4%), $eta$-caryophyllene (5.1%), germacrene D (10.6%), and $alpha$-cadinol (3.0%). The essential oil of C. indicum exhibited stronger antibacterial activity against all oral bacteria tested (MICs, 0.1 to 1.6 mg/ml; MBCs, 0.2 to 3.2 mg/ml) than their major compounds. Furthermore, the MICs/MBCs were reduced to one half $sim$ one sixteenth as a result of the combinations included the essential oil with ampicillin or gentamicin for all oral bacteria. A strong bactericidal effect was exerted in drug combinations. The in vitro data suggest that the essential oil of C. indicum with other antibiotics may be microbiologically beneficial and synergistic.