Flowery assembly of hexagonal close-packed (hcp) cobalt nanoplatelets was fabricated in high yield via a simple one-pot method without any surfactants. The process involved the reduction of the precursor of cobalt dimethylglyoximate with hydrazine in a basic medium. Thus-prepared cobalt flowers exhibited a ferromagnetic characteristic with the coercivity of 308 Oe at room temperature, much higher than that of bulk counterpart.