本文之目的在探討兩項與債務契約有關的問題：沒有租稅津貼的情況下，企業為何舉債？債權人為何對企業經營沒有控制權？藉著不對稱訊息下的契約選擇理論，文中以簡單的模型解釋了這些普遍的現象.企業如需對外籌資以營運，而投資人不了解企業主之管理能力，本文證明了舉債優於發行股票.若企業主欲保持經營控制權，發行股票將顯示出自己管理能力較差，為免投資人不願投資，惟有舉債.如果經營控制權可以商議，則債卷與股票將共同存在於市場中.此時管理能力較強的企業主將發行債卷，但自行保持經營控制權；管理能力差的，將發行股票並讓出經營權，但可享有較佳的報酬.從模型中可引申如下推論：股權較集中的企業通常有較高的負債比例；而企業合併若有經營者更換的現象，亦將提高負債比例.這些推論與一些實證研究結果，頗相符合.

The use of debt has been a continuing research subject after the irrele-vance argument made by Miller and Modigliani. Two issues about debt con-tracts are explored in this paper: why firms use fixed-payment debt contract for external financing if it is Zrrelevant, and why debtholders have no control right. A simple explanation is offered to study the reason of this widely ob-served arrangement. The explanation relies on the theory of self selection from competing contracts under imperfect information. If the investors do not know the managerial ability of the firm's owner, the use of debt financing is shown to be generally better than the use of external equity financing. We find that the debt contract either drives the equity contract out of the mar-ket by revealing the bad managerial ability of the firm's owner, or is used in a self-selection scheme together with external equity contract. In this latter case, the debt contract is preferred by the firm's owner with better manageri-al ability, and the debtholder receives no control right of the firm. The im-plication of the model is consistent with the substantial use of debt in the merger and acquisition activities, and the resulting increase of the acquired firm's market value. Whenever a firm is taken over by a better management, debt issues will replace the equity issues. Meanwhile, it also contributes to our understanding of high leverage in capital structure for closely held firms. If the owner believes he/she can manage better, debt issues will be used for external financing.