The present study examined whether gratitude mediated the relationship between online social support and online altruistic behavior, and whether this mediating process was moderated by social identity. 442 college students from three universities in Hubei province completed anonymous questionnaires regarding online altruistic behavior, online social support, gratitude and social identity. The results showed that : ( 1 ) social support predicted online altruistic behavior positively and (2) the positive association was mediated by gratitude...