Background: Expressed emotion is one of the key factors resulting in relapse of psychiatric disorders. Expressed emotion is a critical, hostile and emotionally over-involved attitude that relatives have toward a family member with such disorder. Since stress from the expressed emotion leads to relapse in person with a disorder, the present study aimed to investigate the effect of mental training based on the acceptance and commitment on the regulation of expressed emotion in families of patients with bipolar disorder. Materials and Methods: This quasi-experimental study was carried out on families of patients with bipolar disorder referring to Kargarnejad hospital (Kashan, Iran). The purposive sampling method was used to select the participants. As an intervention acceptance and commitment-based psychoeducation of the families was held in 10 sessions. Expressed emotion questionnaire was completed by relatives pre- and post-intervention. Results: The results showed that the level of expressed emotion reduced after the intervention in families of patients with bipolar disorder (P<0.001). A significant reduction was observed in the level of expressed emotion in all four subscales (negative attitudes, tolerance/expectations, harassment and intervention and emotional response) (P<0.001). Conclusion: Family psychoeducation based on the acceptance and commitment can be effective in the regulation of expressed emotion in families of patients suffering from bipolar disorder.