The aim of this study is to investigate the change process of early childhood teachers' formative assessment practices via collaborative action research. Formative assessment contributes to children's development and learning process and also guides teachers on instructional decisions. Although research emphasizes the importance of formative assessment, teachers encounter difficulties while implementing it in their classrooms. Three early childhood teachers participated in this qualitative study, and the data were gathered through interviews and observations. The findings about the teachers' change processes revealed that despite the difficulties experienced; teachers began to conduct systematic observation, document children's development and learning, and use assessment data for their further plans. The transformative power of action research in changing teachers' practices is highlighted in this study.