Economic liberalization, globalization of enterprises, mergers, acquisitions and deregulation has caused changes in the business environment and financial reporting requirements. The purpose of the paper is to identify some of the characteristics of corporations that are associated with, and the probable implications of, the quality of corporate disclosure in Iranian companies. In Iran although the content and form of annual reports is affected by certain minimum requirements of the corporation law, the accounting standards committees and the stock exchange for listed companies, there is still a whole host of information that may, or may not, have some influence over the level of disclosure. This paper investigates informational efficiency and the determinants of the extent of the disclosure index of corporations. The results of annual reports survey on a set of Iranian corporations are used to assess the relationship between corporate characteristics and the extent of disclosure measured.