Structural evolution of the ultra-thin cobalt layers grown on amorphous carbon by DC magnetron sputtering were studied in detail by transmission electron microscopy and low-angle X-ray diffraction for a range of the cobalt thickness from 1.5 nm to 4.6 nm. It was shown that atomic structure of cobalt layers was amorphous at the layer thicknesses below 2 nm, an amorphous matrix with embedded nuclei of the crystalline phase with in-plane size of 1-2 nm in the thickness range from 3 nm to 32 nm, and polycrystalline with the randomly oriented HCP cobalt grains at thicknesses over 4 nm. Increase of the cobalt thickness from 32 nm to 4.6 nm led to growth of the cobalt grains with in -plane average size up to similar to 70 nm by the normal grain coarsening process. Transition of the atomic structure of cobalt from the isotropic amorphous state to the anisotropic crystalline state in the thickness range of similar to 2 similar to 4 nm was accompanied by deterioration of the magnetization vector direction within the ferromagnetic domains due to high magnetic anisotropy of HCP lattice of cobalt. (C) 2016 Elsevier B.V. All rights reserved.