We have developed an original method allowing to determine the elasticity constants of thin crystalline films deposited on substrates, which combines X-ray diffraction and in situ tensile testing. This technique has been successfully applied to measure the Poisson's ratio in tungsten thin films (150 nm) and molybdenum sublayers (8 nm) of a Mo/Ni multilayer. This paper gives the principles and experimental requirements for the Young's modulus determination.