A new method for study the elastoplastic property of polycrystalline material is proposed. The neutron diffraction was used to measure the lattice strains in two-phase austenoferritic steel under external load applied "in situ". The evolutions of the internal stresses and of the critical resolved shear stress during tensile uniaxial test were predicted by elastoplastic models. Calculations based on these models were compared with the results of mechanical tests and diffraction data. Finally, the parameters characterising elastoplastic deformation were determined for both phases of duplex steel.