Abstract: Based on the rule of variance composition and uncertainty defined as the square root of variance, the expression of composite standard uncertainty of elastic modulus measured by tensile test was derived, which can be used to evaluate the combined standard uncertainty of the elastic modulus. The evaluation results of an example show that the evaluation results of formula (G. 4) given in appendix G of ISO 6892-1:2019 was obviously lower. Because formula (G. 4) did not include the uncertainty item caused by the error allowed by the accuracy level of force measuring equipment and that of extension measuring instrument. It is suggested that appendix G should be modified accordingly.