This study aimed to evaluate the effectiveness of group–Acceptance and Commitment Therapy (ACT) on the intensity of pain, catastrophizing it and pain-associated anxiety in patients with chronic pain. The research design was quasi-experimental with pretest-posttest and one-month follow-up with control group. The statistical population was all individuals referring to psychological clinics, physical medicine and health centers in Isfahan city who were diagnosed to have chronic pain disorder in 2014. Out of 67 registered patients, 30 patients who had the research criteria were selected by the convenience sampling method and were randomly assigned into two groups each with 15 members. The subjects of the two groups all responded to Chronic Pain Acceptance Questionnaire (CPAQ), Pain Intensity Scale (PIS), Pain Catastrophizing Scale (PCS), and Pain-Anxiety Symptoms Scale (PASS-20). The experimental group received 8 90-minute sessions of Group- Acceptance and Commitment Therapy once a week. The data was assessed in 3 stages of pretest, posttest, and follow-up and was analyzed by multivariable covariance analysis. The results showed that the experimental group had a significant improvement in pain intensity, pain catastrophizing, and pain-associated anxiety (P<0/01) and this improvement was consistent after one-month follow-up. It can be said that Acceptance and Commitment Therapy can be effective on experience of pain of patients with chronic pain.