Friction pair selection for sealing face is a complex issue and no general specification has been proposed yet. The pair of mechanical face sealing materials, as a typical friction pair, shall be selected based on sealed media. The pair of sealing materials involved high hardness materials (such as cemented carbides) and soft materials (such as impregnated graphite and Babbitt alloy). A wide range of materials was available, performance of materials varied a lot and intrinsic nature was not understood. The paper aims to propose a method and provide basis for selection and surface design of friction pair materials. Limitations of plastic index from G-W model were analyzed. Hardness matching of friction pair was considered, system hardness of a friction pair was used to substitute hardness of softer material in the plastic index from G-W model. Meanwhile, modified adhesive theory (in which real contact area could be increased by friction force) was applied to modify the plastic index from G-W model, so as to adapt to complex contact conditions such as friction and introduce friction coefficient. The modified plastic index can be discriminated easily, and it can reflect the hardness matching of friction pair and correlation with wear rate. Generally, non-superhard materials with high plastic index are of higher wear rate; and the high plastic index is in direct proportion to high wear rate.