In this paper, a probabilistic analysis is performed in order to illustrate the effect of uncertainty in G-g and D-g curves on the ground seismic response of a given soil profile. The objective is to propose a methodology to generate a probabilistic modulus degradation curves. This procedure permits to take into account easily a considerable variation about the mean curve of shear modulus degradation and damping ratio in different kinds of soils. Once the methodology presented it is used to study the seismic response of a site profile. The Monte Carlo simulation technique is used to obtain the uncertainties in the seismic response.