PURPOSE: The study is to investigate the effects of tea tree oil gargling on oral cavity micro-organism growth and on the perceived discomfort of patients receiving chemotherapy. METHODS: A nonequivalent control group non-synchronized design was used to determine the effects of tea tree oil gargling on oral cavity for 20 second after using it for one week, twice a day. The sample consisted of two groups of patients receiving chemotherapy : 19 patients in experimental and 20 patients in control group. The instruments used in the study were Oral Assessment Guide(OAG), a measure of perceived symptoms on oral cavity, and a test of oral mucosal micro-organism culture. The data were analyzed using chi-square test, repeated measure of ANOVA, and Pearson correlation coefficient. RESULTS: There was no significant difference between the two groups in micro-organism culture test of oral mucosa. The experimental group showed a lower number and fewer kinds of micro-organisms than the control group. CONCLUSION: It is considered that use of tea tree oil is effective in infection control of the oral cavity.