In this work, we present hydrothermal microemulsion as a novel technique to synthesize cobalt nanorods with hcp structure at a relatively low temperature. The average size of the Co nanorods synthesized for 15h is about 10nm in diameter and 260 rim in length. These nanorods could spontaneously self-assemble into two-dimensional (2D) square-shaped frames or dense multilayer nanosquares assisted by a magnetic field. The formation process and the corresponding mechanism of the novel self-assembled nanostructures are discussed. (c) 2005 Elsevier B.V. All rights reserved.