Cobalt ultrathin films with nominal thicknesses varying between 2 and 10 nm have been grown by DC sputtering process on MgO(0 0 1) substrate at room and high temperatures (650, 750 degrees C). Transmission electron microscopy (TEM) has been used to study the crystallographic structure, strain and morphology of the Co films. The TEM images demonstrate the three-dimensional growth of cobalt at all temperatures, with only fcc structure in low thicknesses, and cobalt islands with hcp and fcc structure at greater layer thicknesses when the temperature is below 650 degrees C. At 750 degrees C cobalt islands present only fcc structure for the whole studied thicknesses. The Co islands started to coalesce at nominal film thicknesses greater than 2 nm with pyramidal shape for hcp grains and flatter mesa shape for fcc grains. Quantitative analysis of high resolution TEM micrographs reveal that whatever the temperature and thickness the Co lattice is relaxed. (C) 2010 Elsevier B.V. All rights reserved.