Abstract: In this paper, the earthquake ground motion is simulated as a zero-mean Gaussian stationary process. On the basis of Newmark’s sliding block idea, a probability analysis model is developed for evaluating the earthquake-induced permanent deformations occured in earth and rockfill dams. The numerical results obtained by Monte-Carlo approach show that the probability distribution of the permanent deformation follows the normal or lognormal distribution. A large amount of calculated data are analyzed and synthesized, the emperical formulae are presented to predict the parameters of the probability distribution and of the average acceleration power spectra of the sliding block. An example of the risk analysis of permanent deformation is given.