The effects of runner surface roughness on the performance of a Francis turbine have been studied experimentally. The performance data were obtained for seven various type roughnesses from 0.007 to 1.0 mm in protuberant height. Preliminarily, the frictional resistance of enclosed rotating disks with respective roughnesses was measured for laminar and turbulent flows, in order to relate these different roughnesses to hydraulic frictional resistance. Through these disk-friction tests, the roughness of these surfaces could be expressed by the equivalent Nikuradse sand-grain diameter. The following conclusions may be drawn from this experimental study. (1) The turbine efficiency dercreases in proportion to the frictional resistance of its equivalent sand roughness. (2) With an increasing roughness, the maximum efficiency point shifts to the lower speed and the lower discharge. (3) Concerning the admissible roughness, the Schlichiting's formula is in good agreement with this experiment.