According to the literature, there are two approaches about the correlation between tax avoiding activity and firm value. The first approach is based on value enhancing, according to which the tax avoiding activity is important for capital market players, petal market players it porches about ... and they consider this as value enhancing activity. The second approach is the agency theory, based on which the vague and complex nature of tax avoiding activities allow managers to hide bad news, in order to achieve their opportunistic goals. On the other hands, the role of corporate governance mechanisms is conducting and controlling the organization operation in order to maximize the interests of shareholders. Therefore, in this study we first examine the theory governing the Iran capital market in relation to tax avoidance and firm value and then the effects of corporate governance index on this relation is examined. To do this, a sample of 96 firms of the listed companies in Tehran stock exchange was selected. The result of this study shows that, there is a positive correlation between tax avoiding activities and firm value. This result confirms the value enhancing theory. Besides, The samples were then classified into groups of good corporate governance and bad corporate governance, in use of piece regression the effect of corporate governance on the relationship between tax avoidance and firm’s value, was investigated. The results indicate that corporate governance have a positive effect on the relationship between tax avoidance and firm’s value.