In modern society, it is a common practice to secure a debt/obligation by the establishment of mortgage; however, some issues pertaining to the law of mortgage still need to be discussed by a more in-depth analysis. This paper undertakes a brief survey about the advantages and/or the disadvantages under the legal system of mortgages among the American Law, the Japanese Law, and Taiwanese Law to articulate issues such as the debt/obligation before mortgage but secured by mortgage after the date of the mortgage, the agreement of mortgage for real estate outside the chain of recording system, and the security interests for overdue interest or liquidation damage under the mortgage of original debt/obligation.