Partnership is one of the key factors to determine supply chain performance, but selecting optimization partners is the precondition for establishing a good partnership. This thesis introduced benchmarking thinking to process of partner selection. It proposed to establish a virtual benchmark partner which has whole optimization selection indexes, and then applied case-based reasoning to measure similarity degree by grey associate degree analysis, and then a descending order of partners can be gain. This method can be applied not only on selecting supplier but also on selecting subcontractor selection by general contractor.