Hexagonal cobalt microspheres composed of ultrathin nanoflakes were obtained via a hydrothermal reduction route by using the metal complex cobalt bis (4-pyridine carboxylate) tetrahydrate as a precursor of cobalt. The diameter of these spheres is about 2 mu m. The thickness of the cobalt nanoflakes is about 10 nm along the easy magnetization axis [0 0 1], which is approximate to the critical size (12 nm) for single-domain behavior in Co. The hysteresis loop of the sample shows an interesting soft magnetic behavior with low coercivity (24.5 Oe). it is noticed that the coercivity is much lower than that of the hcp cobalt nanoflakes reported previously. The peculiar magnetic properties may result from both the anisotropy and the self-assembly manner of the nanoflakes. (C) 2008 Elsevier B.V. All rights reserved.