The X-ray diffraction method has been applied to determine the internal stresses in two phases austeno-ferritic steel during uniaxial tensile test. The elasto-plastic deformation model was used to predict internal stresses and critical resolved shear stress for the both phases. The model calculations were successfully compared with the information obtained from the shift and broadening of diffraction peak. Finally, the parameters characterising elasto-plastic deformation for duplex steel were determined.