The microstructural changes occurring during cooling from 300 to 100 K in a 0.2 μm thick polycrystalline Pb film deposited on a Si3N4 substrate were studied by a combination of transmission electron microscopy and X-ray diffraction technique. The tensile strain induced in the film upon cooling due to the thermal expansion coefficient mismatch between the film and the substrate was observed to be relaxed by dislocation glide. Most of the dislocations were observed to glide across the grains on 111 planes that are inclined at an angle of ∼ 70 deg to the film surface, and on 111 planes that are nearly parallel to the film surface. All the dislocation motions are confined in each grain by the surface oxide, the substrate, and grain boundaries. Some observations suggested that these dislocations emanate from grain boundaries. At 100 K, the density of dislocations introduced in the grains with diameters larger than ∼0.6 μm was found to be roughly constant (about 1010/cm2), while no dislocations were observed in grains smaller than ∼0.6 μm. The observed dislocation density can account for the amount of strain relaxed which was measured by the X-ray diffraction technique. It was also found that almost all the dislocation glide events involved in the thermal cycling process are reversible. This explains a previous X-ray diffraction result that no work hardening effect was observed in Pb films during the thermal cycling at low temperature. The yield stresses of Pb films as determined by the strain measurements are about three times higher than those expected by a simple dislocation pinning model. Based on the dislocation motion observed in this work, the yield stresses of the films were re-evaluated as a function of film thickness and grain size using an energy criterion model. This model took into account the effects of the surface oxide and substrate on dislocation © 1982 American Society for Metals and the Metallurgical Society of AIME.