Thermal stability of ultrafine grained (mean grain size 150 nm) copper prepared by high pressure torsion was studied by means of positron-lifetime spectroscopy correlated with transmission electron microscopy. The microstructure of the material studied is strongly inhomogeneous. The grain interiors with low dislocation density are separated by distorted regions with high number of dislocations. We have found that positrons are trapped at dislocations inside the distorted regions and in the microvoids situated inside the grains. Calculations of the lifetime of a positron trapped at a microvoid as a function of its size were performed to obtain information about sizes of the microvoids. Abnormal grain growth, when isolated recrystallized grains grow inside the deformed matrix, takes place from 160°C. From 280 to 400°C recrystallization occurs. Strongly inhomogeneous spatial distribution of defects does not allow application of the simple trapping model. Therefore a model of positron behavior in the ultrafine grained materials was developed in the present work. The model takes into account inhomogeneous spatial distribution of defects and allows for determination of dislocation density, concentration of microvoids, linear size of coherent domains, and volume fraction of the distorted regions. Moreover using this model it was possible to determine the activation energy of the recrystallization.