This study aimed to explore the timing and strategies of teacher participation in children's play and the factors which have a bearing on teacher participation. This study used qualitative research and conducted observation of natural situations. The samples were preschool teachers in an elementary school's affiliated kindergarten in Hualien. The children were observed while playing at learning areas during free play time. In order to enhance the depth of the research, data were collected from observations of play, interviews with the teacher, and examination of on-site documents. The results of qualitative analysis revealed that preschool teachers usually intervene in children's play actively. Preschool teachers used different strategies depending on the different behavior of children play. As for whether or not to participate; these depend on the teacher's educational philosophy, degree of familiarity with the children, and the overall situation in which the play is occurring. Moreover, each of these factors influences one another, and thus need to be considered together.