Purpose: This study aims to investigate the effects of a peer gatekeeper training program on existential spiritual well-being, interpersonal relationship with friends, and suicide probability for middle school students. Methods: The participants were fifty students who were at a vocational middle school in D city from october 13, 2012 to november 10, 2012. To test the effects of the peer gatekeeper training program, the participants were divided into two groups in the way of matching distribution, an experimental group(24) and a control group(26). Data were analyzed by Chi-square test, t-test, and Fisher exact probability test using the SPSS/WIN 12.0 program. Results: After attending the peer gatekeeper training program, significant differences between the experimental group and the control group were found in existential spiritual well-being(t=2.80, p=.007) and interpersonal relationship with friends scores(t=2.67, p=.010). However, there was no statistically significant difference in suicide probability between the two groups. Conclusion: The results indicate that the peer gatekeeper training program enhance existential spiritual well-being and interpersonal relationship with friends of middle school students. Therefore, this program could be a good strategy that teachers and community mental health nurses can use for suicide prevention of middle school students.