Bullying has emerged as a major social problem recently. The purpose of this study was to find out the effect of bully prevention education on mental health of sixth primary school students. The subject of this study were sampled among sixth grader students (60 experimental group, 60 control group) in primary school, K city. The study was carried out three weeks from 20th November to 7th December in 2000. First, pretest of two groups on mental health. Bully prevention education was practiced 40 minutes at a time, at intervals of 2-3 day`s six times during 2 weeks. Last of all, post-test of two groups were done three days later. The instruments used for this study was a questionnaire asked for mental health. The bully prevention education program consisted of make a friend, understanding bully, role play, warm friendship. OHP film and VTR and paper were used for the education. The experimental group met six 1-hour session(40min./hr.). PC-SAS package programs was used to analyze the data along with χ^2-test, t-test and feedback from students after bully prevention education program were additionally analyzed. The result of this study were as follow; 1) The first hypothesis The first hypothesis was accepted. The experimental group showed more changed in the self scale score than that of control group (t=-6.4643, p=0.0001). 2) The second hypothesis The second hypothesis was not accepted. A significant difference between two groups were not shown in social support(t=-1.6063, p=0.1109) 3) The third hypothesis The third hypothesis was accepted. The experimental group showed more changed in the mental pathology than that of control group (t=3.0861, p=0.0026) 4) The feedback from students after bully prevention education were analyzed. Student who were asked "what if they were bullied?" said that "I will commit suicide" with 68.3%, "I may be sad and lonely" with 18.9%. This result shows when the participants were bullied, they got a shock. They responded as follows, of they are bullied after the prevention education. "I will ask and correct my defect actively." "I will help the students who are bullied." I felt self-confidence by praising friends and by being praised. Bully prevention education have a positive effect on mental health of primary school students. The study showed that bully prevention education was effective in mental health promotion of primary school students. Based on the result of this study, we can recommend tat school-based bully prevention education should be done regularly by school-nurses for induced change of bully knowledge.