A few years ago, some authors have observed by X-ray diffraction analysis in metallic multilayers with small period an in plane expansion combined with a large perpendicular expansion. These trends differ from prediction based on simple continuum elasticity theory. This unexpected result has been the starting point of an experimental development in our laboratory for determining the Poisson's ratio in such systems. Applying the $\sin^{2}\psi$ method on film-substrate set which is elastically deformed in an X-ray diffractometer, it is possible to extract the Poisson's ratio of the film. In this paper, we first detail the theoretical principles of the method and, we show its application on 150 nm thick tungsten films elaborated by ion beam assisted deposition on duralumin substrates. The obtained results demonstrate the feasibility of the method and its good precision.