Abstract: The current limitations of uncertainty evaluation for the proof strength at non-proportional extension Rp0.2 of metallic materials were discussed. By analyzing the process of determining the non-proportional elongation force value in the standard GB /T 228-2002, three uncertainty components which were introduced by extensometer gauge length and extensometer measurement and base line determination were unified into a single model, combined into the standard uncertainty introduced by extensometer. The problem of that there was no mathematical model in three uncertainty components evaluation introduced by extensometer was solved. The new uncertainty evaluation method for Rp0.2 was established. And the uncertainty of an example was calculated by the method.