Introduction: Separation anxiety disorder (SAD) is characterized by developmentally inappropriate and excessive anxiety concerning separation from home or those to whom the individual is attached. Parent-child interaction therapy (PCIT) is a treatment approach that integrates traditional and behavioral techniques in the treatment of behavior problems in young children. The purpose of this study was to investigate the effectiveness of parent-child interaction therapy (PCIT) in reducing of the separation anxiety of children. Method: This study was carried out as semi experimental and pre test-posttest doing with a comparison group. The Sampling was made using in available method. At first three regions (1, 2 & 6) out of 12 regions in Mashad was chosen from which 11 kindergartens were randomly selected. According to CSI-4 the children that had the symptoms of separation anxiety was 83. At the end, two groups of 12 children randomly selected and the mothers were also randomly assigned to two groups of experiment and comparison group. The experimental group had 5intervention sessions of parent-child interaction therapy (PCIT). The comparison group had no PCIT. At the end of treatment, a post test was taken from both groups. Results: The results of covariance analysis showed significant reduction in symptoms of separation anxiety in experimental group compared with control group (p