We demonstrate the formation of highly ordered twofold symmetric rectangular nanoparticle superlattices by the slow evaporation of solvent from colloidal dispersions of oleic acid/oleylamine-coated Fe3O4 nanoparticles on a water surface. These superlattices covered regions of micrometers in size without any noticeable disorders or defects, with size controlled by the amount of oleic acid added to the colloidal dispersions. The superlattices were transformed into arrays of nanowires by subsequent calcination. The peculiar nanoparticle assemblies are discussed in terms of the cooperative self-assembly of nanoparticles and fatty molecules during the slow evaporation of solvent.