This study integrates theories of diffusion and social identity to conceptualize the diffusion of Sustainability Report (SR) as a result of a firm's identification with its reference groups. Specifically, we first hypothesize four different sources of external stakeholder pressures driving the diffusion. Next, we argue that the source of external stakeholder pressures has a differential effect on the adoption of SR for firms that claim their identity on sustainability management. For firms with organizational identity claims, in-group stakeholder pressure will amplify whereas out-group stakeholder pressure will dampen the adoption. We test our theory using an event-history analysis of 675 publicly traded firms in Korea during the period of 2003-2010. The results show that all four sources of external pressure serve as mechanisms through which SR spread in Korea. More importantly, we find support for the moderating role of organizational identity claims in the effect of external pressures. We discuss how organizational identity matters in the diffusion of corporate social initiatives along with implications for policy makers.