Cobalt films of different average thickness (t(Co) = 0.15-4.8 nm) have been deposited on amorphous alumina (a-Al2O3) at different temperatures (T-s = 130-600 degreesC), with the aim of forming Co nanoparticles of well-defined size and shape. Two deposition techniques have been used: thermal evaporation (TE) of a Co filament at the pressure P = 10(-10) mbar and pulsed laser deposition (PLD) from a Co target at P = 2 x 10(-6) mbar. The structural properties of the Co films have been investigated by using transmission electron microscopy (TEM), electron diffraction (TED) and high resolution (HREM). Co nanoparticles are found at high temperature, T-s = 350 degreesC and above, in the films prepared by both techniques. In the early stages of growth, they are nearly spherical and at further stages they become oblate and the height/base-diameter (h/phi) ratio decreases when the diameter increases. The differences between the films prepared by the two techniques appear principally at low deposition temperature, T-s = 270 degreesC and below: worm-like shape of pure Co are found in the films grown in the TE chamber and quasicontinuous layer films containing Co and Co oxide are found in the films grown in the PLD chamber.