This study analyzes the factors responsible that can significantly affect Banking's Net Interest Rate in Commercial Banks located in Indonesia, Thailand, and the Philippines. Data were collected from 35, 10 and 13 commercial banks in Indonesia, Thailand, and the Philippines from 2012 to 2020. The Simultaneous Equation Model was used to analyze the macroeconomic factors and banks' specific characteristics towards Loan and Time Deposit rate which will ultimately affect NIM. Several uniqueness of this study comparing with previous research are: this study using simultaneous equation model to find determinants factors affecting NIM, this study apply additional variables (Z score and HHI) on loan rate equation, & this study consider to apply both real rate & nominal rate when most of the research only use nominal rate. The result showed that Macroeconomic factors, such as the Inflation rate, significantly affect NIM. Meanwhile, the specific bank factors that contribute to lowering the NIM in each country are quite varied. Therefore, regulators in each country need to consider these important variables when making decisions on lowering the net interest rates by banks to enhance social welfare. Keywords: Interest Rate Spread, Commercial Banks, Financial Institutions and Services.