The dilemma of disclosing one's illness experiences in real life has led to a proliferation of online health communities. It is worth exploring the nature of such communities. Drawing on the community of practice (CoP) theory, this article explores how members enact online health communities by studying the support group "Philosophical Treatment of Depression," one of the largest online depression communities in China. The findings show that participants enacted the CoP by constructing negative individual identities and positive collective identities, participating and engaging in the common enterprise, and building a shared repertoire. Constructing negative individual identities and positive collective identities enhanced group cohesion and clarified community goals. Participation and mutual engagement facilitated members' sharing of illness experiences and seeking/providing support in the community. Sharing treatment and coping experiences and tactics contributed to a shared repertoire that sustains the community. This study provides insights into understanding the nature of online health communities. It also has practical implications for social support to people living with depression.