Abstract:The inhibition of chitinooligosaccharide (COS) against Phytophthora nicotianae var. nicotianae and the control effectiveness of COS on tobacco black shank caused by the oomycete were investigated in this paper. And the dynamic activities of superoxide dismutase (SOD), peroxidase (POD), polyphenol oxidase (PPO), phenylalanine ammonialyase (PAL) and chitinase in the tobacco plants treated with COS were measured to explore the mechanism in bio-control effects of COS on the disease. The results showed that in vitro, COS had not inhibition against the mycelium growth of P.nicotianae var. nicotianae on lima bean agar. However, on the condition of artificial inoculation and pot cultivation in greenroom, COS and its combination were effective for the control of the disease, and the control effectiveness was 57.81% in the treatment with COS solely. Next were the control effectiveness of Trichoderma+COS+chitin and COS+Trichoderma in which the effectiveness were 53.49% and 52.49%, respectively. The results of enzyme activity tests showed that COS enhanced activities of SOD, POD, PPO and chitinase in tobacco plants in 10 days after treatment in greenroom, with the peak values increased by 96.30%, 136.36%, 10.34% and 9.10% compared to the control respectively, which was consistent with the result of the control test. The results suggested that COS had biological activity to induce and improve resistance of tobacco plants to black shank, with considerable potential in control of the destructive tobacco disease.