Co nanotubes were successfully prepared in the pores of anodic aluminium oxide templates using a DC electrodeposition method. The dependence of the product morphology on the applied potential was studied over the range -1.0 to -3.0 V. The results showed that the wall thickness of the nanotubes became thinner as the applied potential was reduced, and that there existed a critical potential (v (c)) related to the electrodeposition parameters, below or above which the electrodeposition process is dominated by kinetics and thermodynamics, respectively. The formation of nanotubes is the result of kinetics dominating the electrodeposition process. Magnetic measurements showed that the hysteresis loops of the nanotubes were discontinuous. Theoretical analysis suggested the existence of some peculiar magnetization configuration such as a vertical Bloch line, which was responsible for the discontinuity of the hysteresis loops.