Abstract: Surface roughness is a key factor of overland flow velocity and its variation. This study systemically discusses variations of average overland flow velocity V, surface roughness d and discharge q with slope gradient S using data of fixed-bed model tests. Flow velocity was measured on a flume with adjustable surface roughness (d≤10mm), unit discharge (0.04~0.24 L?s-1?m-1), and slope gradient (2°~12°). Results show that when surface roughness increases, the water flow consumes more energy in overcoming resistance and flow velocity is decreased exponentially. Different roughness leads to different main control factors of overland flow velocity. At lower roughness (0≤d≤2.5mm), roughness, discharge and gradient are dominant factors that contribute 19.8%, 44.8% and 27.8% of the flow velocity variance respectively, while at higher roughness (2.5