This study purposed to children who had acquired externalization problem due to parent-child dysfunctional communication to examine whether improving communication between parent and child reduces the children's externalization problem. The following research questions were set: first, changes in the child's and mother's verbal reactions detected during therapy; second, changes in the child's externalization problem. The subjects of this study included three couples of school-aged child and mother. The attended a 50-minute session once a week ten times. Each session sand play was carried out in Boik's and Goodwin's(2000) seven approach sages: 1st stage: creating the world, 2nd stage: experiencing and rearranging, 3rd stage: communicating with the partner, 4th stage: therapy, 5th stage: documentation, 6th stage: transition, 7th stage: dismantling the world. During sand play, changes in the child's and mother's verbal reactions were analyzed sequential analysis. The basic ratio, the individual subject's ratio of occurrence frequency to the entire reactions detected in a session, and the average frequency of reactions of each session were calculated. As for children's externalization problem, parents evaluated children's externalization problem before and after the sessions. The results were compared and analyzed in original scores and T scores. Furthermore according to analysis criteria, qualitative analysis was performed on children's externalization problems detected during session with the reference of a recorded tape, verbatim records, and the therapist's therapy report. The child and mother generated vigorous verbal interactions and expressed their feelings frankly during sand play, As they attend more sessions, the child's and mother's positive sentimental reactions increased. On the other hand, their negative sentimental feelings decreased. The improvement of communication between parent and child through sand play significantly reduced the children's externalization problem. This study is meaningful in that it has proved sand play is effective in improving communication between parent and child.