The development of autonomy is acknowledged as a significant educational goal in language education, but it seems challenging in many schools. One of the reasons for this discrepancy is the lack of professional development addressing teacher beliefs and practices about learner autonomy (LA) as valid educational concerns. However, in teacher education research, limited attention has been paid to language teachers' understandings and practices of promoting LA. Hence, this research project aims to develop in-service teachers' beliefs, understandings, and practices about LA through teacher development workshops. First, an in-depth interview was used to gather five teachers' beliefs about autonomy and teaching practices in secondary schools. Second, two workshops were conducted based on the literature and the initial interview to develop teachers' LA-related beliefs, understandings, and practices. Third, a post-workshop evaluation was conducted through a focus group, the written reflection, and the researcher's self-reflective journals to examine to what extent teachers' knowledge and beliefs about LA have changed and developed. The findings revealed that the online workshops have made making teachers more aware of the concept of LA and its significance in language learning. Their LA-related beliefs and understandings of their teaching objectives, prior learning experiences, and teaching competence influenced their LA-related classroom practices. Some practical suggestions were provided for designing and implementing professional development workshops.