Abstract: In this paper, effect of R_P0.2 due to the errors of extensometer gage and extensometer system was analyzed. Through test analysis on several materials, it was found that the measurement error of R_P0.2 due to strain error depends on the property of materials. For ordinary materials, it could be supposed that the error of R_P0.2 due to error of strain is less than 1%, but for some alloys it is rather more than 1% where a high-grade extensometer must be used to measure these alloys.