The purpose of this case study was to investigate the effect of play therapy on a child with emotional instability. The subject was a 7 -year-old boy, who displayed emotional instability such as temper tantrums, computer game dependence, constipation, oppositional defiant behavior, etc. This study was designed to determine the effectiveness of 40 play therapy sessions. In the beginning phase, the child relaxed in the secure relationship with the therapist. In the middle phase, the child expressed anxiety, aggression, positive emotions and positive self-image. These changes were maintained in the final phase. At the same time, the relationship with the mother and peers improved.