Capital adequacy ratio is considered as one of the most important stimuli in the profitability of financial institutions. The main objective of the present study is to investigate the determinant factors of capital adequacy ratio in Iranian banks. For this purpose, fourteen private and the state Iranian banks during the period 2006 to 2010 are selected for study. Statistical data and other requirements were extracted from audited annual financial reports of selected banks. To examine the determinant factors, the size of the bank's capital adequacy ratio is considered as dependent variable and variables such as size of the bank, share of deposits, loss reserve facilities granted, facilities granted amount, liquidity, debt leverage, return on assets and return on share holder’s equity are considered as independent variables. A multiple regression model using panel data is employed to test the relationship between dependent and independent variables. The results show that the variables such as liquidity and rate of return on assets have a positive and significant effect on required capital adequacy ratio, while the bank size, the facilities granted shares, return on shareholder’s equity, loss reserve on facilities granted and debt leverage have a negative and significant on the capital adequacy ratio. While no significant relationship between the share of deposits and capital adequacy ratio is confirmed.