Austenitic stainless steels have widespread applications in various industries due to their proper corrosion resistance and ductility. The main deficiencies of these engineering alloys is their relatively low yield strength, to improve which the grain refinement mechanism may be used. In recent years, many researches on the grain refinement of austenitic stainless steels by the martensite thermomechanical process have been conducted, in which careful examination of mechanical and thermal variables is time-consuming and costly. therefore, there is a high tendency to model such processes. In the present work, finite element simulations have been carried out to investigate influence of the austenite grain size distribution on the formation of strain-induced martensite (SIM) in a Ni-free austenitic stainless. For this purpose, a finite element numerical model through the software coding Abaqus UMAT was created. Firstly, the macroscopic behavior model in a two-dimensional tensile test was examined and validated. Then, the macroscopic model was developed on the basis of applying grain size distribution to be used for real microscopic structures. In order to create a model with random grains, Voronoi code in Matlab software was used and the real metallographic images were also converted to binary form by Image J software. The results showed that the present model is able to offer formation kinetics of SIM with acceptable accuracy. Numerical results in agreement with experimental data showed that reducing the austenite grain size caused retardation in the formation of SIM. Considering the mixture hardening rule, the variation of volume fraction of SIM was obtained within the grains. At low strains, application of a mean hardening modulus and at high strains, using a mixture hardening modulus, presented more consistent numerical