The purpose of this study was to develop a group play therapy program using unstructured play materials. The group play therapy program was developed following the three steps. First, pre-investigation consisting of literature review and interview has been made to design the therapy program. Second, the goals and contents were determined and initial application and evaluation plans were set. Third, a pilot application of the group play therapy program was attempted and, given the attempts, modifications of the program were further made. Modifications were made particularly on phases of the program, ways to present unstructured play materials, and child-teacher interaction skills. The final group play therapy program using unstructured play materials that was developed was intended to help preschoolers develop their potentials to play, consisted of 9 sessions, and included three development periods--the relaxation period, the tactility experience period, and the creation of play period.