This paper presented a 2D finite element model of AISI 4340 for micro side cutting. JohnsonCook constitutive model and JohnsonCook shear failure model are combined to simulate a complete side cutting cycle for one flute rotation. The flow stress which is calculated from JohnsonCook constitutive model is smaller if the size effect is neglected. Therefore, the strain gradient plasticity model is developed. The chip formation, von Mises stress and temperature fields are predicted. Compare to the JohnsonCook constitutive model, simulation results show that size effect exists in the micro side cutting process and the strain gradient plasticity model can describe it well. Finally, the flow stress from the finite element simulation is compared with theoretical calculation results, which is validated the correctness of finite element model.