本文主要是在探討，處於路徑依賴效用買權架構下，當銀行在同時追求淨權益報酬與規避淨權益風險的情況，銀行最適銀行利息收益率的行為。具體來說，因為在到期前，股權權益的資本投資成本乃為權益持有人銀行股權投資的機會成本和相關資產的價值，因此銀行的淨股權權益可被視為買權訂價的模式。我們嘗試去推導，最適的銀行利差與資本市場利率即權益資金成本的關係。更進一步地，探討在忽略風險下，是否將導致對報酬的高估而承擔過度的風險，並導致銀行體系的不穩定性。 This paper examines the optimal bank interest margin, i.e., the spread between the loan rate and the deposit rate of a bank, when the bank’s preferences include the like of higher net equity returns (equity returns net of equity capital investment costs) and the dislike of higher net equity risks based on a path-dependent, barrier option framework. Specifically, the bank's net equity is priced as a down-and-out call option because equity capital investment costs are explicitly treated as bank equity investment opportunity costs of equity holders and the value of the underlying assets crosses a stated level before the exercise date. A direct implication of this framework is that the objective function is viewed as a down-and-out utility call option. This paper want to see how optimal bank interest margin may be affected by the security-market interest rate, and thus to the equity capital cost. Furthermore, we want to see how this impact on the optimal margin which ignores the barrier and/or the dislike leads to overestimation and how may the bank lead to less prudent and more prone to loan risk-taking the stability of the banking system.