The Manning equation is one of the most widely used formulae for calculating the velocity of shallow overland flow in hydrological and erosion models. Precise estimation of the Manning's friction coefficient (n) is critical to determining overland flow and soil erosion processes. Few studies have been conducted to quantify the effects of sediment load on Manning's n on steep slopes. This study was conducted to investigate the potential effects of sediment load on Manning's n in a flume with a fixed bed, under wide ranges of hydraulics and sediment loads. Slope gradient varied from 8·7 to 34·2%, unit flow rate from 0·66 to 5·26 × 10−3 m2 s−1, and sediment load from 0 to 6·95 kg m−1 s−1. The Reynolds number ranged from 350 to 5899. Results showed that Manning's n varied in both sediment-free and sediment-laden flows ranging from 0·012 to 0·055. The apparent Manning's coefficients of sediment-laden flow were much greater than those of sediment-free flow. The mean Manning coefficient of sediment-laden flow was 51·27% greater than the mean value of sediment-free flow. For sediment-laden flow, Manning's n could be estimated with a power function of unit flow discharge and sediment content. Further studies are needed to quantify the potential effects of sediment load on the Manning's n on erodible beds and in fields. Copyright © 2010 John Wiley & Sons, Ltd.