Abstract: In this study, a new approach has been investigated in solving time-cost trade off problem, because of uncertainties which affect on activity cost. Fuzzy logic theory is employed to consider affecting uncertainties in total direct and indirect cost of a construction project. Non-dominated Sorting Genetic Algorithm (NSGA), a multi objective optimization algorithm, is applied to provide a trade-off between time and total cost. Project manager can also have different non-dominated solutions or pareto solutions which are dependent on his measure of accepted risk through applying α cuts methods in fuzzy logic theory. The proposed model leads the decision maker to select the desirable pareto front solution through acceptable value of α cut.