In the current world, saving energy in the whole society has been a global hot topic in people’s daily life. Built environment is considered as the area which has the most potential for energy efficiency in the Netherlands. For the schools, the behindhand energy facilities on one hand lead to huge energy consumption, on the other hand, they also caused great problems of indoor environment quality in these schools. So that schools are with urgent need for sustainable retrofitting. The energy service company (ESCOs) and energy performance contracting (EPC) is considered as one of the most effective mechanism for achieving energy efficiency in built environment. But the implementation level of it is with a low level in the Netherlands due to lack of awareness and trust. So this thesis developed a decision support tool which aims at helping the users to get a better understanding about the EPC contract and its applicability in sustainable school retrofit project. More specifically, this decision support tool is focusing on the financial part of the decision making part and provides optimization contract design with key contract terms to users to help them in decision making. Firstly a literature review is conducted to identify all the key variables needed in the model. Then system dynamics and net present value (NPV) are employed to represent the financial interest of each party within the ESCO. Then Monte Carlo simulation is used to generate the range values of uncertain variables within the NPV equations. After that, this thesis used non-linear programming to finish the optimization process and get the final result. A case study is employed to verify the usability of this decision support tool. The result suggested that this decision support tool can provide the users with exact result which is expected in the research goal.