Measurements of diamond bulk density change of carbon powder in the annealing of UDD to OLC (293-2400 K) was performed by irradiating the samples with a narrow beam of gamma quanta (137Cs gamma-ray source). The intermediates of UDD to OLC transformation were registered by high resolution TEM. The annealing products were also characterized by true density and specific area adsorption measurements. The structural rearrangement of UDD begins from the surface towards a crystal bulk. UDD annealing proceeds via particles composed of semi-spherical shells enclosed one-to-one with the rest of diamond core and quasispiral carbon shells. The influence of particle size on the temperature of UDD to OLC transformation was found.