Information asymmetry arises when one of the parties to the contract or transaction, possesses more information than the other, provided that the former uses such information effectively when communicating with the latter. The main objective of this study was to examine the effects of information asymmetry on the value of companies. Two techniques i.e. standard deviations of returns (STDRET) and turnover (TURNOVER) were used to determine information asymmetry while the Tobin’s Q was employed to determine companies’ value. The population under study was all the listed companies in Tehran Stock Exchange, of which a total of 72 companies were chosen as the research sample and were studied over the period 2004-2009. Panel analysis technique was used in this study to analyze the collected data. The results of the study indicate information asymmetry affect the value of the companies.