This paper discusses a research strategy which determines the optimum capital structure strategy and sets a reasonable required return of two-dimensional mortgages for borrowers and lenders with trade-off theory. We derived the optimum capital structure strategy of REITs and assess the mortgage risk premiums for lender. The results indicate that the REITs should maximize the value themselves by adjusting the optimum loan-to-value, make the most efficient use of the money. The lenders should seek borrowers who can pay mortgage interest continuously. At the same time, by maximizing mortgage premiums, they can set strategic objectives.