Social impact assessment (SIA) has become a key factor for environmental conflicts prevention. In this study, we conducted SIA using the grey clustering method, which is based on grey systems theory. A case study was conducted on a mining project located in Cajamarca, Peru. Two stakeholder groups and seven evaluation criteria were identified. The results revealed that for the urban population stakeholder group, the project would have positive social impact; and for the rural population stakeholder group, the project would have negative social impact. The method showed quantitative and qualitative results that could help central and local governments to make the best decision on the project. Moreover, the grey clustering method showed interesting results that could be apply to assess other projects or programs from social point of view.