Dental caries is one of the most prevalent dental diseases in Korea and its prevention is very important in orthodontic therapy. For the cleansing action of saliva itself and/or tooth-brushing is lowered in patient with fixed orthodontic appliance, oral hygiene of the patient becomes worse, which provides more favorable environment for micro-organisms. Chlorhexidine, one of the series of bisguanide, has been reported to be strong antimicrobial agent and very effective on Streptococcus mutans. The purpose of this study is to evaluate the possibility of chlorhexidine as a anticariogenic agent in fixed orthodontic therapy. We used the varnish containing chlorhexidine as a main ingredient for the chemical control of salivary S.mutans in patients with fixed appliance therapy. We applied the varnish containing chlorhexidine on the labial and interproximal surface of the teeth before bonding and banding teeth of our patients(N=20) and compared to control group patients(N=20). Before the application of chlorhexidine varnish and four times periodically after the completion of fixed appliance set-up, we sampled saliva of both group patients and incuvate S.mutans for 24 hours. In the culture study of sampled saliva, counting the number of S.mutans colonies, we founded as follows : 1. In patients with fixed appliance therapy, the risk of dental caries increase when it compared to that of preorthodontic treatment; The number of salivary S.mutans increase in patient´s oral cavity. 2. The experimental agent that contain chlorhexidine is effective to reduce the number of salivary S.mutans. 3. For the effect of this agent is not ever-lasting, periodical application is needed, and additional study for economical interval and number of application is needed.