This study examines the impact of corporate governance structure and free cash flow on over-investment on 121 firms listed in Tehran Stock Exchange over the period 2008-2011. To measure over-investment, free cash flow and corporate governance variables based on available information reported on financial statements are gathered and using cross section regression method, different hypotheses of the survey are examined. The results indicate that among corporate governance mechanisms investigated in survey, there is a significant relationship between percentage of non-executive directors and ownership concentration with over-investment. However, there was no meaningful relationship among controlling shareholders and duality with over-investment. Furthermore, a significant relationship between free cash flow and over-investment has been found.