Abstract: The experimental formulas for the frictional resistance in open channel 1-D steady flow are based on one-to-one relationships among the hydraulic parameters. These formulas of frictional losses in steady flow cannot simulate the frictional losses in unsteady flow accurately because the hydraulic parameters reach their peaks at different times and their relationships among themselves are more complex. Therefore, using dimensional analysis, an improved differential structure formula has been developed. The experiment done in this study shows that the improved formula which takes the temporal and spatial variations of hydraulic parameters into account has higher accuracy than that of Chezy formula, and be of practical value.