The characteristics of the roughness coefficient are very important for practical application. Some experiments are conducted to study the variation of Manning’s n with flow depth, mean velocity, and density of vegetation,. An assumed velocity distribution to describe the vegetative flow of submerged vegetation is confirmed by experimental results. The measured velocities in this study seem to have little effect on the curve of n ～ h, and a new linear relationship between Manning’s n and flow depths is observed clearly. According to the arguments that the flow resistance of densely unsubmerged vegetation is dominated by the resistance exerted on vegetations, the influence of the density of vegetation on Manning’s n is estimated. On the basis of the velocity distribution, the n ～ h curve under submerged condition is theoretically obtained from the n ～ h curve under unsubmerged condition. These results are also well confirmed by experimental results and very significant for practical applications.