<p>Background: School bullying has a variety of negative influences on victims and bullies, as well as on bystanders. In this article, we examine psycho-social factors associated with bullying behaviors among Japanese junior high school students for developing an effective bullying prevention program.</p><p>Objective: The aim of this study was to examine the relationships between being bullied, bullying others, responses to bullying as bystanders and psycho-social variables including self-esteem, life skills（social skills, coping skills and decision-making skills）and perceived social support among junior high school students based on the longitudinal data.</p><p>Methods: The participants were 1,528 seventh grade students attending nine junior high schools in five cities from four prefectures. The baseline data were collected between June and July in 2015（T1）, using anonymous self-administered questionnaires and the follow-up survey was administered in February and March in 2016（T2）. Dependent variables included the students’experiences of being bullied and bullying others and their responses to bullying as bystanders since their transition to junior high school at T2. Independent variables were their self-esteem, life skills and perceived social support at T1.</p><p>Results: The participants were divided into four groups, namely “neither victim nor bully”, “victim only”, “bully only”, “victim and bully”, in accordance with their experiences of being bullied and bullying others since their transition to junior high school. According to the results of one way analysis of variance, students who reported both being bullied and bullying others a few times a month or more than once a week since their transition to junior high school tended to show the lowest scores on measures of family-related self-esteem, pro-social skills, problem-focused coping skills and perceived social support from parents and peers. In contrast, students who had never experienced being bullied nor bullying others showed the highest scores on these measures.</p><p>The participants were also divided into five groups, namely “didn’t see bullying”, “joined in bullying”, “watched what was going on”, “walked away from the bullying”, “tried to help victims”, in accordance with their self-reported responses to bullying as bystanders. The results of one way analysis of variance indicated students who joined in bullying tended to show the lowest scores on measures of family-related self-esteem, pro-social skills, problem-focused coping skills, decision-making skills and perceived social support from fathers. In contrast, students who tried to help victims showed the highest scores on these measures.</p><p>Conclusion: These results suggest the importance of enhancing students’self-esteem, life skills (social skills, stress coping skills and decision-making skills) and perceived social support from significant others to help them to avoid being bullied, not bully others and help victims when they witness bullying.</p>