Abstract: The difference of effects of strain strengthening on the tensile mechanical properties of S30403 and S30408 austenitic stainless steels was studied. The results show that with the increase of strain strengthening value, the yield stress of the two stainless steels significantly increase, the plastic parameters obviously decrease, and the ultimate strengths almost unchange. The yield stress of the two materials are basically the same, but there are differences in tensile strengths, plastic parameters and strain hardening exponents. The differences of mechanical properties between the two stainless steels after strain strengthening are related to the dislocation, twin and martensite phase transition. Finally, according to the evolution law of mechanical behavior and strain energy density with the change of strain strengthening value, a modified Hollomon constitutive model considering strain strengthening was constructed, and the accuracy of this model is demonstrated by the experimental results.