Purpose: This study was done to investigate the effects of a Peer Gatekeeper Training Program on self-esteem, interpersonal relationships with friends and suicide probability for high school students. Methods: The recruited participants were forty-three students a vocational high school in G city. The study was done from August 31, 2012to September 25, 2012. To test the effects of the Peer Gate Keeper Training Program, the participants were divided into two groups, an experimental group (22) and a control group (21). The Peer Gate Keeper Training Program was provided to the control group after the posttest was completed. Data were analyzed using x2-test, t-test, and Fisher exact test with the SPSS/WIN 18.0 program. Results: After attending the Peer Gate Keeper Training Program, significant differences were found in self-esteem, interpersonal relationships with friends, and suicide probability scores between the experimental group and the control group (t=3.24, p=.002) (t=2.16, p=.036) (t=-2.95, p=.006). Conclusion: The results indicate that the Gate Keeper Training Program enhances self-esteem and interpersonal relationships with friends and reduces suicide probability in high school students. Therefore, this program is a good strategy that teachers and community mental health nurses can use for suicide prevention in high school students.