This study examines the effect of managerial mechanisms of corporate governance (board and CEO characteristics) on risk taking of Iranian firms listed in Tehran Stock Exchange. Risk taking proxy is idiosyncratic risk (unsystematic risk). Board and CEO characteristic proxies are board independence, board size, CEO duality, CEO tenure and CEO dominance. We used standard deviation of the residual return explained by the Fama-French (1993) three - factor model as a surrogate for idiosyncratic risk. Also we used Leverage, ROA and AGE as control variables. Using a collected sample of 127 listed companies of Tehran Stock Exchange during 2008 - 2013 and using panel data regression, our empirical evidences show that board independence, board size and CEO dominance are associated with idiosyncratic risk, whereas other variables have no influence on idiosyncratic risk.