The rehabilitation of a sewerage system is to maintain its initial designed capacity. Implementing multi-objective optimal rehabilitation plans results in both increase in benefit and mitigation of negative environmental impact.This paper proposed a multi objective optimal model to determine the optimal rehabilitation plans of Sam Son sewerage system, Thanh Hoa province. NSGA-II was utilized to define Pareto solution sets which were trade off solutions among three objectives including minimum rehabilitation cost, maximum service life and minimum traffic impact. As a result, rehabilitation cost contours in relation to service lifes and traffic disruption were established to determine optimal rehabilitation plans.Three trade off plans including the plan A with the minimum rehabilitation cost (12,5 billion VND), the plan B with the maximum service life (78 years) and the plan C with minimum traffic disruption (0 veheicle/hr) were analyzed in more detail to facilitate decision makers. Using the multi objective optimal design helped to quantify social costs of rehabilitation costs.