Successful implementation of formative assessment in classrooms relies on teachers' formative assessment literacy, which is typically supported by professional development programs. This study reports on the impact of a professional development program on a group of in-service primary teachers in Hong Kong. The authors investigated the effectiveness of the program in developing teachers' formative assessment literacy and how far the program assisted them in addressing the challenges they faced when implementing formative assessment in classrooms. With a mixed-method design, quantitative data were collected with a pre- and post-program survey (N = 27); qualitative data were collected with open-ended survey questions and a focus group with six participating teachers. The results provide evidence of the program's success in developing teachers' formative assessment literacy and equipping them with the necessary skills to address the challenges in implementation. The features of professional learning communities embedded in the program contributed to the positive impact of the program.