While Task-based Language Teaching (TBLT) has been widely researched and accepted as a potentially powerful teaching method in language education around the world, research examining the teachers’ beliefs about the impacts of a training on TBLT to date has remained limited. In Vietnam, studies on this topic were scarce (Nguyen & Jaspaert, 2021). This study thus investigates the Vietnamese English as a Foreign Language (EFL) in-service teachers’ beliefs about the impact of a TBLT training program. To collect data, classroom observations followed by semi-structured interviews were conducted with five volunteer teachers. The findings showed that the teachers generally had strong beliefs in the usefulness of the TBLT training program for professional development, and their willingness to pursue TBLT teaching. Although there exist barriers hindering this TBLT training program, many teachers expressed interest to undergo more TBLT training in the future. The findings in this study are expected to provide some insightful pedagogical implications for developing a TBLT training program for in-service teachers in the similar context of language education in Vietnam and other EFL settings.