This study examines the effect of new mandatory disclosure policy, Government Regulation no 47/2012, on Indonesian environmental disclosure practices. The sample consists of 249 companies listed in Indonesia Stock Exchange from all industries except trading and financial sectors. Tha data is based on annual reports and sustainability reports from 2011 to 2013 which were available in the public domain. Year 2011 was the year before the policy was issued, 2012 was the issuance year, and 2013 was the year after the issuance. Comparing environmental disclosure practices at the year before and the year of the mandatory policy being issued, there were significant increases in terms of GRI index based information being reported; the number of words used to report environmental issues based on GRI; carbon emission index being reported; and the number of words used to report carbon emission aspects. In year 2011, only 13.95% companies were disclosing based on GRI, but then the amount increased to become 25.25% in 2012 and 30.90% in 2013. Based on detail analysis, it was found that the significant increases occur not in the companies with high carbon emission, but in the companies with moderate or low carbon emission. This study supports legitimacy theory as described by Suchman (1995) where in order to gain legitimacy, the role of social audience in legitimacy dynamics should be addressed. In this case, the issuance of new mandatory regulation has increased participation of moderate and low carbon emission companies in environmental disclosure practices where previously the issues used to be of interest for companies with high carbon emission only.