This paper addresses the question of how taxation affects the cost of capital of firms and value of firms as measured by Tobin’s q. We construct a Real Business Cycle model and derive our original unlevered q on an after-tax basis, by removing financial tax shield effects in order to disentangle real operating profitability of firms and their financing decisions. Our model is an extended version of the two-sector general equilibrium model originally developed by Christiano and Fisher [1] and can incorporate both corporate and individual taxation. The unlevered q-value is derived from our general equilibrium solutions and some comparative static results are demonstrated with model predictions. In an empirical section of the paper, we find that the data support these model predictions, and thus they rationalize the use of our unlevered q. Our result possesses important policy implications for financial managers of the firms in correctly identifying firms’ true profitability aside from corporate tax shields as well as for the tax authority in changing the regulatory corporate tax rates.