Cobalt–carbon nanotubes composite (Co–CNTs) is synthesized through a facile hydrothermal route. SEM and TEM characterizations reveal that the Co–CNTs composite contains abundance of carbon nanotubes connected by cobalt spheres and some of the CNTs are filled with metallic nanoparticles or nanorods. A series of electrochemical measurements show that the adding CNTs can remarkably enhance the electrochemical activity of the Co, leading to a notable improvement of the discharge capacity and the cycle performance. The practical maximum discharge capacity of the active Co is 495mAhg−1 after deducting the weight contribution of CNTs, which is about 280mAhg−1 higher than that of pure Co. The electrochemical reaction mechanism can be attributed to the dissolution–precipitation mechanism of Co in alkaline solution. And the functions of the CNTs are to improve dispersion of Co particles, increase contact area between Co and alkaline solution and promote the charge-transfer reaction.