Evolution of microstructure and morphology of Co3O4 particles in the model systems during reduction in hydrogen was studied by transmission electron microscopy methods. Based on SAED and HRTEM results we found that the degree of reduction of Co3O4 strongly depends on the particle size and morphology, which are determined by the pretreatment conditions. Preferential epitaxial growth of CoO and Co phases on Co3O4 during reduction was deduced from HRTEM images.