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This paper presents a novel strain measurement technique for determining Poisson's ratio of micron-thick polycrystalline TiN films. We developed a new compact tensile tester operating with X-ray diffraction (XRD) equipment in order to evaluate Young's modulus, Poisson's ratio and fracture strength of TiN films. TiN films having the thickness between 1.5 μm and 2.0 μm were deposited onto 9 μm or 24 μm-thick single crystal silicon (SCS) specimen by rf reactive magnetron sputtering. Young's moduli of TiN films deposited at Ar pressure of 0.7 Pa and 0.9 Pa were found to be 290 GPa and 240 GPa, respectively. Poisson's ratios of the films were, for the first time, determined to be 0.36 and 0.27 by out-of-plane normal strain measurements using XRD during tensile tests. The change in the mechanical properties of TiN films with Ar pressure should be attributed to the change in the film density. This technique proposed can be useful for accurately measuring Poisson's ratio of micron-thick single- and poly-crystalline MEMS films.