In this paper, the occurrence of cavitation on horizontal axis marine current turbine blades has been investigated by numerical methods. To implicate the effects of cavitation on turbine performance, a momentum blade element method was used in conjunction with a boundary element method which adopts the section cavity lengths as inputs and computes the lift and drag coefficients of cavitating blade sections. Distribution of cavitation along the blades of marine current turbines was also modelled using a vortex lattice method. In order to assess the capability of the methods, marine current turbines tested previously under certain conditions were analyzed and the results were compared with the experimental data available in literature. Satisfactory agreement validated that the distribution of cavitation along the blade and the length of cavity at each blade section can be predicted adequately for a cavitating marine current turbine. Using the methods presented in this study, up to 30% theoretical loss in generated power is predicted for the particular case of a model turbine subjected to cavitation under specific test conditions.