This study investigates high school EFL teachers' perceptions of the necessity and effectiveness of online teacher professional development (PD) to their teaching practices during the COVID-19 pandemic. One hundred and fifty teachers having experienced online teacher PD workshops by the MOET participated in the study. The study employed a mixed-methods approach with a concurrent research design. Data were collected and analyzed through questionnaires and interviews. Overall, the findings have indicated that the teachers perceived online PD workshops as a beneficial format to help them improve their pedagogical skills to teach online. The findings have showed that while the teachers felt confident and motivated as a result of attending the online PD workshops, there were factors affecting their online teaching practices (e.g., poor internet speed, lack of technological skills and technology literacy of both teachers and students, and lack of proper training). On the basis of the findings, practical implications have been made so as to increase the sustainability of online PD activities for EFL teachers in similar contexts.

Key words: EFL teachers, online teacher professional development, professional development; COVID-19 pandemic, Vietnam.