Ordered arrays of monodisperse cobalt ferrite (Co Fe2 O4) nanocrystals with highly controllable spherical or cubic shapes have been synthesized via solution-based thermolysis of an intimately mixed Co2+ Fe2 3+ -oleate precursor. The evolution from spherical to cubic morphology is achieved by simply changing the precursor concentration, thereby controlling the nanocrystal growth rate. Magnetic studies indicate that the saturation magnetization is independent of the shape and is solely determined by the size of the nanocrystal. However, the coercivity does exhibit a small shape dependence, primarily resulting from the influence of surface anisotropy. © 2008 American Institute of Physics.