Micro water grid (MWG) is an emerging concept for decentralized water management system for residential, industrial, commercial, and institutional buildings. MWG achieves specific local goals, such as reliability, diversification of water sources, and cost reduction. Unlike micro electricity grid, MWG has not been well-established, leading to a problem of initial design and operation. Thus, a MWG system requires a “smart” infrastructure that allows autonomous control and regulation. In this study, a decision support system (DSS) was designed to select optimum combinations of water treatment unit processes in MWG. To begin, key questions on water treatment in MWG systems were identified. Then, the knowledge-based system was developed based on the inputs from experts in related research fields and the prediction of treatment efficiency from theoretical model. A series of experiments were carried out to obtain the model parameters for each unit process (i.e. media filtration, microfiltration, GAC, nanofiltration, chlorination, UV). The flow chart for decision making was created and revised upon considering virtually all possible situations in MWG. As the final step, a web-based system was developed and implemented. Although it is a first version of DSS for MWG, it has potential for an increase in the efficiency of water treatment under various situations. ACKNOWLEDGEMNET This research was supported by a grant (12-TI-C01) from Advanced Water Management Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.