Results: In all studies except one, supervisor training included provision of basic knowledge of psychosocial factors at work; In most studies from Japan the training focused on providing skills to communicate with subordinates (such as "active listening" skills); in one study from Sweden, they put more focus on redesign and improvement of psychosocial work environment. A onetime session was applied for most studies; multiple sessions with two-week intervals during one year were provided in one study. One RCT used a small group discussion; two RCTs used a web-based training method. Most studies used three-month follow-up. A significant overall effect of supervisor training was found on mental health among subordinate workers (Effect size, -0.13; 95%CI, -0.26 to 0.00; p=0.05). A marginally significant effect was observed on job control (Effect size, 0.100; 95%CI, -0.01 to 0.21; p= 0.06). No significant effect was observed on job demands, supervisor or coworker support, or work performance (p>0.05). Conclusions: The meta-analysis indicated the effect of supervisor training on mental health among their subordinate workers, while the effect size was small. Supervisor training may enhance worker mental health through improving their job control. Supervisor training does not seem to have an effect on increasing work performance.