Car accidents ending up with death have often existed in the country's roads, and children who witnessed the death of their family members are highly prone to psychological diseases including posttraumatic stress disorder. Since this disorder may affect children's intellectual growth and mental performance as well as causing mental damages, contributing to the treatment of this disorder takes on special importance. For this reason, the research was conducted with the aim of determining the effect of group play therapy in a behavioral-cognitive manner on the reduction of posttraumatic stress disorder among children aged 6 to 10 who witnessed the death of their dear members in car accident. the method was a quasi-experimental study. At first, children referred to child psychiatry clinics in Tehran, were identified by Rutter's Child behavior questionnaire in order to evaluate behavioral symptoms and Yule's scale for evaluating the symptoms of posttraumatic stress disorder, which was completed by their relatives. Subsequently, 20 children with this disorder were selected as sample randomly and divided into two 10-member groups randomly, experimental and control. The experimental group was treated by group play therapy in a behavioral-cognitive manner in 8 sessions on a weekly basis, and the control group received no intervention. Data were statistically analyzed by covariance analysis. The results indicated that group play therapy based on a cognitive-behavioral technique during posttest reduced the symptoms of posttraumatic stress disorder (P<0.01). group play therapy on cognitive-behavioral basis can be effective in reducing the symptoms of posttraumatic stress disorder in children witnessing the fatal accidents of their beloved relatives.