The global expansion of the use of online communities, including social networking sites, necessitates a better understanding of how people self-disclose online, particularly in different cultures. In a scenario-based study of 1,064 respondents from the United States and China, we aimed to understand how self-disclosure is affected by communication mode (face-to-face vs. online), type of relationship and national culture. Our findings show that national culture interacts with communication mode and type of relationship to affect the extent of self-disclosure. Our analysis also suggests that peoples' disclosure depends on characteristics of the relationship, e.g., closeness and openness. Our results shed new light on how online communities might be designed for users in different cultures and for intercultural collaboration.