The aim of the study is to explore the effect of firm size and quality of disclosure on the cost of common stock. The cost of common stock as dependent variable, quality of disclosure and firm size as independent variables, financial leverage and book value to market value, and systematic risk as control variables have been reviewed. The time span of the study was a ten year course starting from 2003 to 2012, and selected sample consisted of 1090 year-firm. The research method is a correlational descriptive method, and a multivariate regression method was used to test hypothesis. Results indicated that there is a significant negative relationship between firm size and cost of common stock as well as between quality of disclosure and cost of common stock.