The seismic assessment of existing dams is normally affected by system complexity and uncertainties. A probabilistic method is then considered in the present work to effectively manage such uncertainties regarding structural data and external actions, and also to account for several potential failure mechanisms, whose definition and numerical identification appears to be another open issue. The method has been applied to the case of Kasho concrete gravity dam, for which strong motion data are available. A finite element discretization of dam body, foundation rock mass and reservoir water is employed to effectively simulate the dynamic response. Results of the seismic assessment are presented in terms of fragility curves for the whole system and for each failure mechanism.