Risk assessment of landfil systems using Monte Carlo simulation in the prediction of groundwater contamination underneath a landfill insulated by clay liner is presented. The advantage of applying stochastic modeling is that uncertainties associated with various design parameters can be taken into account in the prediction of pollutant transport through the clay liner. A model based on a pollutant transport model considering advection, dispersion and sorption is developed in this study. The design parameters with their corresponding distribution forms are fed into the simulation model. The probability of the contaminant concentration in the groundwater underneath the landfill exceeding a predetermined value is used for risk assessment.