In the paper we present the synthesis of Co hollow nanospheres by surfactant-assisted hydrothermal method at mild condition. The XRD pattern indicates the sample is hexagonal close-packed (hcp) Co with cell constants a = 2.512 angstrom and c = 4.102 angstrom. The Co hollow nanospheres have the outer diameter of about 50-200 nm with the thin wall of 10-20 nm. Room temperature magnetic measurement of the Co hollow nanospheres demonstrates its enhanced ferromagnetic property. The surfactant CTAB might play a vital role in the formation of the hollow Co nanospheres, and simply adapting different reaction temperatures can control the size of these hollow nanospheres. The possible formation mechanism was also discussed. (C) 2008 Elsevier B.V. All rights reserved.