Manning’s n is a variable with time and space under flow’s effect; and it is influenced by many relevant physical factors. Therefore, the decision of Manning’s n is difficult. For practical purpose, it is necessary to establish a standard procedure to estimate Manning’s n for the sake of river planning and design. This project studies the relevant physical factors of Manning’s n and their extent of influence to Manning’s n. In order to establish a standard procedure to estimate Manning’s n, this project get the fundamental information by investigating the riverbed around the floods, and discuss the influence on Manning’s n due to related physical factors by employing numerical models. Because the data of water level, cross section and riverbed material in Tou-chien River is complete, and the convenience of data investigating, the calibration and validation of the numerical model in this project use the data from Tou-chien River. The project develops procedure for estimating Manning’s n and let us understand the factors influencing Manning’s n including riverbed particle size, and physical factors such as Froude number and unit stream power. For the purpose of practical application, the project will consider other possible affecting factors next year. In order to get the Manning’s n conveniently, the procedure for estimating Manning’s n will be refine also future.