The vertical distribution of lattice strains in low-pressure chemical-vapor-deposited polycrystalline silicon (poly-Si) films grown on SiO2 was studied by energy-dispersive grazing-incidence X-ray diffraction with synchrotron radiation white X-rays. The results revealed that a 5 nm thick poly-Si film consisting of nanometer-scale islands had tensile strain originating from poly-Si/SiO2. interface stress. Thicker films indicated not only strain relaxation but also compressive strain at thicknesses greater than 20 nm. Transmission electron microscopy showed lattice defects including dislocations. This leads one to conclude that compressive strain is caused by insertion-type dislocations induced by grain-boundary formation and subsequent bending in these boundaries.