The shear strength properties of municipal solid waste (MSW) are of special importance when evaluating the stability of landfill slopes. Geoenvironmental engineers are well aware of the existence of many sources of uncertainties associated with shear strength parameters of MSW due to various reasons. The significant uncertainties associated with the shear strength and shear stresses render deterministic modeling potentially misleading. The traditional engineering approaches like method of slices used for evaluating MSW slopes are frequently questionable as they do not adequately account for uncertainties included in analytical modeling and natural variability. In order to quantify the slope stability precisely by taking into account the variability, the reliability based design optimization (RBDO) framework is presented. The mean and standard deviations associated with unit weight, cohesion and angle of internal friction of the MSW are taken into account in the probabilistic optimization. Reliability analysis is performed using first order reliability method (FORM). A limit state function is formulated against sliding slope failure using Spencer method of slices. The influence of coefficients of variation (COV) of stability number and friction angle on critical center coordinates and reliability index is presented in the form of charts. © ASCE.