Large-area erosions such as rust and obvious cavitation were found on the surface of the guide vane in Three Gorges hydropower units.A numerical explanation of the cavitation is given in this article.At first,based on the characteristic performance curves of the prototype hydro-turbine supplied by ALSTOM together with the actual operating conditions,an operating point is chosen for numerical analysis using the Reynolds-Averaged Navier-Stokes (RANS) equations.The flow passages from the inlet of the spiral case to the outlet of the draft tube are included in the computational domain.The results show that the static pressure on the guide vane surface is much higher than the critical pressure of cavitation.Secondly,a tiny protrusion on the guide vane surface is considered and the problem is simplified to a 2-D problem to study the local detailed flow near the guide vane surface.The protrusion is 0.5 mm in height and is 5.0 mm in width.On the basis of the results of RANS simulations,the 2-D problem is studied using the Large Eddy Simulation (LES).It is shown that there exists a region in which the static pressure reaches a level below the vapor pressure of the water.Thirdly,a cavitation model is included for the 0.5 mm protrusion case and another tiny pit case,with a tiny pit 0.3 mm in depth and 1.0 mm in width.The results show that vapor bubble exists at the protrusion entrance and the pit exit as the low pressure regions.