Factors that determine the level and variation in disclosure have been a matter of considerable interest and importance to policy makers and the financial reporting community. Existing studies have not well established the impact of institutions on corporate disclosure because of their macro-level analysis. This thesis investigates the association between firm-level institutional factors and the level of mandatory disclosure in annual reports of Nigerian listed companies. It argues that accounting standards provide the definition of legitimate methods for use in presenting financial statements, and the level of mandatory disclosure reveals organisational commitment to these standards. The thesis uses the Oliver (1991) and Greenwood et al. (2011) institutional framework to identify factors that determine the level and variation in mandatory disclosure. The thesis sampled 100 firm-years across eight industries over three regulatory regimes. The self-constructed measure of mandatory disclosure is based on the Nigerian national accounting standards, which provide guidance for presenting financial statements prior to 2012, and on the IFRS, for first time adopters of IFRS with a financial year-end of 2012/2013. Based on Oliver’s framework, the result indicate that the level of mandatory disclosure is significantly and positively influenced by legitimacy, legal coercion, and voluntary diffusion, however, it is significantly and negatively influenced by economic efficiency, uncertainty, interconnectedness and dependence. These results suggest that Nigerian listed companies confront greater number of factors that encouraged resistance to disclosure in annual reports. Based on the Greenwood et al.’s framework the result indicate that strong regulatory regimes significantly and negatively influenced variation in the level of mandatory disclosure while organisational field, organisation structure, ownership and identities significantly and positively influenced variation. These results suggest strong regulatory regimes reduced variation in disclosure while organisation structure, ownership and identities increased variation in mandatory disclosure. The results provide alternative explanation on determinants of mandatory disclosure.