In this paper we study how a firm’s capital structure choice affects informed trading of its securities in the secondary markets and consequently, the information efficiency of its security prices. We identify two new factors as the potential determinants of the firm’s optimal capital structure policy: the liquidity premium caused by informed trading and, perhaps more importantly, the improved operating efficiency due to information revelation from its security prices. We show that, from these two perspectives, the optimal debt level is achieved at the point where there is no informed trading in the bond market and the informed traders are just about to trade in the bond market. Thus, the cost of debt financing differs in nature from that of the existing models. This has very different implications for the significance of the cost of debt financing and for financial system design. Our model can also explain the relative trading volumes in debt and equity markets.