Erosive wear of hydro turbine runners is a complex phenomenon, which depends upon many parameters and which leads to a decrease of the performance in time and/or in extreme cases to the rotor mechanical failure. Consistently, the study of this wearing process is an important step to improve the impeller design, to avoid or minimize the rise of extraordinary maintenance. In the present paper the cavitation mechanics of a Pelton turbine was investigated using CFD analyses. A Pelton affected by pitting cavitation was taken as test case. The Pelton geometry was modelled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS) multiphase analyses. The homogeneous approach was used to describe the multiphase flow composed by water, water vapour and air. Numerical results discriminated the vapour production processes during the cut in of the bucket on the water jet. The design and the part load flow rates were analyzed and the cavitation process compared. A simple procedure to identify the locations of higher damage risk was presented and verified on the test case runner.