This thesis has done a mathematical model of flow across the Saphue rapids by using the InfoWorks RS model simulation of flow in Mun river using hydrologic data from August 1 to November 30 2012. Which consists of a low altitude line of the area, Data level at Saphue rapids, roughness coefficient data of manning, cross section picture of river the number of cross section picture of river which study all 56 front cut. Total distance education based on the Mun river about 80 kilometers. Create a model to run model type Steady State and Unsteady State. The upstream uses flow rate data from M7 water measuring stations and downstream uses water level data from water measuring stations, Pak Mun dam. And perform the calibration accuracy using the water level measuring at M 11B stations. Found that the roughness coefficient of the Mun River from M7 water measuring stations to Pak Mun dam there is a rough coefficient in the range of 0.025 to 0.04. For a bank use a roughness coefficient in the range 0.03 to 0.12. Calibration modeling using the water level which data actual on August 1 to November 30 2012 found that the error only 0.227 percent. From the simulation can explain the behavior of flow across the Saphue rapids that when the flow of water from the upstream to the Saphue rapids the water level will be reduced gradually until the end of 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.