This study explored the impact of the online professional development program of in-service English teachers joining Teacher Certification Program 2019. Teacher professional development is often seen as the key to powerful education improvement. Therefore, teachers are expected to experience continuous professional development to keep abreast of the endless change taking place in the education routines. Challenges arose when teachers had to be enrolled in an online professional development program which was regarded as a compulsory program conducted by the government as a requirement to get a standardized license in teaching. This study took the form of a qualitative study employing two instruments: questionnaires and interviews. The findings revealed that after joining the online program, teachers confirmed that the impact mainly covered on teachers’ knowledge, teachers’ practice, student learning outcomes, and teacher efficacy.