This study investigated professional learning taking place in a teacher-created online community. In particular, this aimed to explore how teachers at different levels of participation learn in an online community. The results showed that teachers usually began as observers, reading others' postings and using contributors' teaching resources, and moved to collaborators, posting their comments on teaching resources and discussing their problems with other teachers, and then to contributors, sharing their teaching resources with other teachers and providing information, advice, and help to other teachers. As they participated in the online community at different levels, teacher learning took place in multiple ways including learning through tryout, collaborative problem solving, and critical reconstruction. Yet, not all teachers moved toward full participation in the online community. The majority of teachers remained as observers and their lack of participation in the negotiation of meaning constrained teacher learning. For the online community to be a fertile ground for teacher learning, more emphasis needs to be placed on the collaborative construction of knowledge and the transformation of shared practice, rather than the transmission of knowledge and the dissemination of good practice.