Although the ultrasonic nonlinearity has been studied to evaluate the material degradation, it does not tell intuitively about the degree of material degradation so that the yield strength obtained from the destructive tensile test is still widely used. This study proposed a new algorithm to measure the linear and nonlinear elastic moduli based on the linear and nonlinear ultrasonic techniques, which is reduced to estimate the 0.01% offset yield strength by through the reconstruction of the tensile stress-strain curve in the form of quadratic polynomial within the elastic range. In order for demonstration, the heat-treated Al6061-T6 specimens were prepared, and the 0.01% offset yield strengths were estimated by the proposed algorithm to compare with those obtained by the tensile test. Results showed good agreement. This method can be used to evaluate the degradation of yield strength quantitatively in isotropic material.