The introduction of micromechanical parameters in the description of the mechanical behaviour of polycrystalline materials is getting more and more important. In the interest of the study of inter- and transgranular stress and strain heterogeneities inside an assembly of grains, the multicrystal is introduced. The complete microstructure is known which makes possible an experimental and numerical approach which Lakes into account the complete microstructure. The modelling is done by a finite element simulation meshing the grain morphology. The experimental validation is done by x-ray diffraction and optical microscopy. The multicrystal is elaborated on the base of a coarse grained Nickel based 600 alloy. A long heat treatment close to the melting temperature allowed to get into the regime of grain growing. In the finite element calculations the grains are considered as perfect single crystals with only one crystallographic orientation. Naturally, during the elaboration process materials develop sub-grain structures with slight misorientations called small angle grain boundaries or grain mosaicity. The quality of the crystal can be described by the mosaicity. Depending on elaboration parameters, the intergranular misorientations can be distributed more or less randomly. Due to the local distribution of mosaic blocs, the heterogeneity of the measurement depends on the observation scale. The use of optical microscopy allows the determination of plastic activate zones. The observation of heterogeneities inside these zones needs finer measurement techniques such as x-ray diffraction. The measurement of misorientations by rocking the sample around a crystallographic axis allows to quantify the misorientations. This is even more interesting during plastic slip where, due to the anisotropy of the crystal, the mosaicity develops anisotropically, too. Finally, the microscale parameters like the mosaicity are investigated by keeping in mind the influence on the local stress and strain evolution. The comparison to the finite elements simulation is of great interest in relation to the choice of micromechanical parameters which do not contain anything comparable to a mosaic function.