To determine the effectiveness of ergonomic training on postural habits and computer operators’ psychosocial mediating variables based on the transtheoretical model (TTM).A prospective randomized controlled trial.Departments of two universities in Qazvin, Iran.One hundred and thirty-four of operators, mean age 31.0±7.2 years, who worked at a computer for a minimum of 20h per week. The subjects were divided into intervention (n=67) and control (n=67) groups.The intervention group received a TTM-based intervention involving eight 2-h sessions, which included 45–60min stage-matched ergonomic counseling, ergonomic behavior training, and a package of training materials comprising a facilitator's handbook and a handout regarding computer ergonomic guidelines (Ergo-Guidelines).Ergonomic knowledge, stage of change, self-efficacy, pros and cons, preventive behavior and rapid upper limb assessment (RULA) scores were measured at baseline, 3- and 6-month post-ergonomics training.Although both groups were homogeneous in all variables at baseline, the intervention group showed significant improvements in stages of change (P<0.001), ergonomic knowledge (P<0.001), pros (P<0.05), cons (P<0.05) and self-efficacy (P<0.05). A significant decrease in RULA score was observed for the intervention groups; however, intervention did not significantly improve RULA.Results from this study provide evidence that TTM-based ergonomic training among computer operators can improve postural risk factors for musculoskeletal disorders (MSDs).