This study was conducted to define the role of urban Material Recovery Facilities (MRFs), which are just one step in the flow of recyclables from collection through manufacturing recycled products. It is assumed that urban MRFs usually function the following four processes, removing nonrecyclables, separating, reprocessing and reducing the volume of recyclables. Factors affecting those functions were chosen to define the actual roles for Seoul's MRFs by surveying several public MRFs, private MRFs and manufacturers. Picking out nonrecyclables is the most important role of Seoul’s MRFs because 15 to 30% of total recyclables moved into public MRFs is nonrecyclables. Passive separation, which is to separate recyclables step by step and to collect the residue as nonrecyclables, is more effective to remove nonrecyclables in recyclable stream. Separating incoming materials into 13 groups is preferable, and those groups, like mixed paper, PE/PP, PS/other plastics, clear bottles, amber bottles and green bottles, require inevitably further separation in the following markets. PET, third plastic package, EPS, aluminum cans, tin cans and metal products reveal that costs related to stock and shipment were lowered by their volume reduction. To prevent environmental problems and the hindering further separation, it would be reasonable for public MRFs not to do material processing, e. g. crushing and bending recyclables, as a their roles.