In developing countries, policy-makers at all levels are confronted with the problem of prioritizing different projects which needs to be done in a systematic method for the optimal allocation of funds. The current study is focused in addressing the above-mentioned problem for urban storm water drainage networks, and it aims to address two key questions: ‘Which drainage projects should be tackled first?’ and ‘Does the project definition structure influence the prioritized projects?’ The north-eastern part of Tehran is selected as the case study and the first question is addressed through a prioritization method based on particle swarm optimization (PSO) algorithm. Three different criteria including investment costs, flood damage costs, and exposure costs are considered. It is concluded that the multi-objective approach can better handle the problem in hand. To deal with the second question, two levels of project definition are investigated. The results reveal that the project definition structure has a clear impact on the objectives and its enhancement can lead to better system performance in terms of objective functions. This finding implies that despite the current trend in the city of Tehran, project definition structure should receive appropriate attention before the results are evaluated in a subsequent prioritizing framework.