The Young's modulus and Poisson's ratio of reduced thickness layers are generally unknown whereas simulation of mechanical behavior of thin film/substrate systems or stress determination by X-ray diffraction cannot be dune in an accurate way without the knowledge of these values. In this paper, we present three types of experiments which are used in our laboratory for determining elastic constants in polycrystalline thin films elaborated by ion beam sputtering: A vibrating reed device, X-ray tensile testing and AFM buckling geometry analysis. Results obtained for metallic 304L stainless steel thin films are given.