Introduction: Posttraumatic stress disorder is a set of clinical symptoms that occurs following exposure to life-threatening events, and affects a variety of psychological variables. The aim of the present study was to evaluate the effectiveness of prolonged exposure therapy on social support and the severity of symptoms in patients with post-traumatic stress disorder. Methods: This was a quasi-experimental study with pretest, post-test and a two-month follow-up design and a control group. Using stratified sampling method, 30 patients (15 subjects in each group) with posttraumatic stress disorder referred to Farabi Psychiatric Clinic in Kermanshah City selected and they were randomly assigned. The Mean age of the sample group was 31. 9. The study inclusion criteria was having PTSD based on clinical interview. The study exclusion criteria included having psychosis symptoms, drug addiction. The data were collected by structured clinical interview (SCID-I), social support scale (MOS), and the fifth edition of the post-traumatic stress disorder index (PCL-5). Data were analyzed by repeated measures ANOVA test. Results: The results showed that prolonged exposure therapy was significantly effective in increasing social support (F=2/66, P<0/01) and reducing the severity of symptoms (F=42/19, P<0/01) among the patients with post-traumatic stress disorder. Conclusion: These results indicate that prolonged exposure therapy can alleviate the symptoms of the patients with post-traumatic stress disorder.