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A method for quantifying uncertainty in conceptual-level design via a computationallyefficient probabilistic method is described. As an example application, the investigated method is applied to estimating the propellant mass required by a spacecraft to perform attitude control. The variables of the design are first classified and assigned appropriate probability density functions. To characterize the attitude control system a slightly-modified version of Subset Simulation, an efficient simulation technique originally developed for reliability analysis of civil engineering structures, is used. The proposal distribution aspect of Subset Simulation is modified vis-à-vis the original technique to account for the general characteristics of the variables involved in conceptual-level design. The results of Subset Simulation are compared with traditional Monte Carlo simulation. The investigated method allows uncertainty in the propellant required to be quantified based on the risk tolerance of the decision maker. For the attitude control example presented, Subset Simulation successfully replicated Monte Carlo simulation results yet required significantly less computational effort, in particular for risk-averse decision makers. Nomenclaturelognormal random variable with parameters μ and σ m number of Subset Simulation levels N number of Subset Simulation samples N c number of Markov chains developed at each simulation level; N c = p 0 N