A deformation mechanism map was constructed to study the mechanisms of strain relaxation in lead thin films which were deposited on oxidized silicon wafers at room temperature and which were then thermally cycled between room temperature and liquid helium temperature. The stress level, which was calculated from the strain measured by an X-ray diffraction technique, was plotted on the map. By comparing the calculated and experimental stress levels the following observations were obtained. In the cooling process the strain was relaxed rapidly in a field of dislocation glide mechanism for films of greater than 0.2 μm thickness. In the heating process most of the strain was again believed to be relaxed by the glide mechanism. For a film 0.5 μm thick the stress (after the primary relaxation was completed) was found to be (1-1.5) × 109 dyn cm-2 for the cooling process and (0.17-0.24) × 109 dyn cm-2 for the heating process at temperatures around 200-280 K. Slow secondary relaxations were observed after the primary relaxations were completed. The measured compressive strain relaxation rate at room temperature was very close to the rate calculated on the assumption of grain boundary diffusion creep. This suggested that the secondary relaxation mechanism of compressive strain was grain boundary diffusion creep at temperatures near room temperature. These suggestions were supported by scanning electron microscopy observations in which dislocation slip lines were observed inside grains and hillocks were observed on grain boundaries. © 1978.