The purpose of this study is to figure out the cross-cultural differences between American and Chinese college students in self-disclosure on social media sites. In particular, it will examine the influence of culture (especially collectivism and individualism dimensions) in both content (breath) and depth of self-disclosure on Facebook and Renren. Also, this study aims to ascertain gender differences in the influence of self-disclosure (breath and depth). This study applied collectivism versus individualism, one of Hofstede's cultural dimensions, and self-disclosure from social penetration model to examine the cultural differences between U.S. and Chinese college students in self-disclosure on social media sites. The results showed that there exits the nationality difference in self-disclosure's width and depth, implying that Chinese students self-disclose more in width and depth than American college students. Gender differences in self-disclosure's width and depth were also found and supported by previous study. Females prefer to self-disclose more in width and depth than males in SNSs. But the gender differences across nationality were only partially supported by the study; only U.S. females tend to self-disclose in more width than U.S. males.