Polycrystalline approaches are more and more used to characterise the mechanical behaviour of material with the interest to take into account the mechanical properties of the material constituants. In this case, for a better understanding of the polycrystalline material mechanical response, mesoscopic and microscopic parameters must be clearly identified. Our purpose is also to characterise the mechanical behaviour of a cast duplex stainless steel, containing 30% ferrite and 70% austenite, using X-ray diffraction. The mechanical state can be determined using X-ray diffraction technique which enables to measure strains in each phase separately. The measurements are performed at the grain scale because the studied material exhibits coarse grains of up to one millimeter. In this case, only few crystals are irradiated by the incident X-ray beam and the classical sin(2)psi method for stress determination can not be applied to this material. An adaptation of the single crystal measurement method to large grains materials is used. Stresses so determined correspond to second order stresses. The measurement in each phase has successfully been applied to follow the stress state evolution during an in-situ tensile test. Several grains, with different crystallographic orientations were studied. For each one, the stress tenser was determined in the two phases under different macroscopic loading in elastic and plastic domains. A modelling of the mechanical single crystal behaviour is developed and our pourpose is to identify the parameters of the single crystal behaviour law using X-ray single crystal stress analysis during in situ tensile test.