Factors leading to successful professional learning communities (PLCs) have been widely discussed in the West, but little is known about how/whether teachers' beliefs contribute to PLCs in the Chinese context. This qualitative case study aimed to investigate teachers' beliefs about teacher learning in PLCs and their influence on collegial learning activities in two departments of a Hong Kong secondary school. Semi-structured interviews and observations were employed to collect data. The findings indicate that teachers in the Chinese Department believed that learning could be achieved in PLCs and their practices of collegial activities were conducive to teacher learning. Conversely, teachers in the English Department did not believe that interacting with colleagues would foster learning and their collegial activities yielded limited accomplishment in teacher growth. It is suggested that teachers' beliefs play an important role in facilitating or impeding reform initiatives on teacher development. Implications for teacher education and further studies are discussed.