This Thesis has done a mathematical model. The flow of water across the Saphue rapids by using Inflows RS model in the simulation of the flow in the Mun river. Using the hydrological data on 1 August to date 30 November 2012 which is a relatively current event information of area studies. The results of modeling can show the water level. The result of the simulation can be displayed in table form and graph relationships. Found that the flow rate from the recording and the flow rate from the calibration model is a little different. The roughness coefficient of the Mun river from the station M7 water measurement to Pakmun dam there is a rough coefficient in the range of 0.026 to 0.04.For a bank use a roughness coefficient in the range 0.03 to 0.14.When calibration reliability of mathematical model can describe the behavior of water flow across the Saphue rapids. Found that when the flow of water from upstream reaches Saphue rapids. The water level will be reduce gradually until the end the Saphue rapids. The Manning’s roughness coefficient will result in changes in the water level and the accuracy of the study depending on the resolution of the terrain data and include the accuracy of hydrological data.