The behaviour of unsaturated soils is governed by the soil-water characteristic curve (SWCC). The suction in unsaturated soil has a significant impact on the stability of infinite slopes. The measured SWCC and suction values in the soil are inherently variable and uncertain. Due to these uncertainties, the results generated by conventional deterministic approaches render unrealistic factors of safety of the slope. Therefore, it is highly appropriate to develop a reliability based design optimization (RBDO) for stability analysis of unsaturated infinite slopes. The effect of variability associated with fitting parameters, af, mf, and nf on the reliability index of infinite slopes is presented It is noted from the present study that the maximum values of COV's for af, mf and nf are 489.2, 412.5, and 297.4% respectively for different types of soils. The reliability analysis is presented based on RBDO using four different models to predict the unsaturated soil shear strength. The mean and standard deviation associated with suction, cohesion, unit weight, and internal angular friction are considered for the probabilistic optimisation. A performance function is formulated against sliding slope failure. The influence of the coefficients of variation (COV) of unsaturated soil parameters on the reliability index is also discussed. © 2018 American Society of Civil Engineers (ASCE). All rights reserved.