[en] We have grown, by sputtering, Co films on (1 1 1) Si substrate, with the aim to find the critical thickness of the reorientational transition of the magnetization from the plane to out of the plane as the hcp axis is oriented perpendicular to the film. Stripe Domains (SD) by Magnetic Force Microscopy, characteristic signature of Perpendicular Magnetic Anisotropy, have been found only in samples grown in some series while samples grown with the same growth parameters do not show stripe domains, indicating an in-plane orientation of the magnetization. These apparently controversial results will be explained in terms of the system fundamental parameters, magnetization, exchange stiffness constant and perpendicular magnetic anisotropy of the sample, which play a crucial role especially in Co films