Evolution of the inner microstructure of uniaxially pressed copper powders was investigated by in situ X-ray microtomography. Experiments were carried out at the European Synchrotron in Grenoble, France. Sintering was performed under reducing atmosphere at 1050 °C. Qualitative and quantitative information from the 3D images acquired along the whole sintering cycle were analyzed. From that, macro- and microstructural features of the evolving microstructure were obtained. We found that large pores can be created during sintering and then they can grow during the whole thermal cycle by reducing the initial relative density of the sample by 10 %. The effect of the pore shape and the heterogeneous distribution of the pore volume fraction inside the sample are responsible for the behavior observed during sintering. The heterogeneous deformation measured is controlled by the evolution of the porosity.