In this paper, we have used the finite element micromagnetism theory to simulate the magnetization (M) reversal behaviors of cobalt nanotubes with different lengths (L = 400, 600, 800 and 1000 nm). The inner radius and outer radius of nanotubes are 50 nm and 100 nm respectively. The simulation results show that all the nanotubes exhibit significantly anisotropic behaviors: the easy magnetization axis is along the longitudinal direction. The coercivity values are found dependent on the dimensions of Co nanotubes. For the nanotube with a length of 400 nm, M reversal processes along the Z axis starts from the inner wall and propagates from the inside toward the outside. However, for other nanotubes, M reversal processes start from both ends of nanotubes. When the magnetic field (H) is applied along the hard axis, magnetic moments gradually change their directions into the opposite directions. The variations of the total Gibbs free energy have been calculated to support the observed differences in reversal processes.