Cobalt iron oxide nanotube arrays with various wall thicknesses were prepared by atomic layer deposition in porous anodic alumina template. Nanotubes uniform in thickness and homogeneous in composition can be obtained with aspect ratios on the order of 50. These nanotubes have a polycrystalline spinel structure. Both the mean grain size and the grain size distribution increase with the tube wall thickness. Correspondingly, their magnetic properties (remanence and coercive field) also have a strong dependence on the thickness for measurements carried out at 300 K. This dependence is attributed to the superparamagnetic behavior of the grains that constitute the nanotube.