Single-crystal ZnWO4 nanorods can direct the self-aggregation of the amorphous nanoparticulates and promote the crystallization and transformation process of the amorphous nanoparticulates derived from a simple supersaturation precipitation reaction in a nanorod/amorphous nanoparticle coexisting system under refluxing conditions or mild hydrothermal conditions. In this system, the obvious nanorod-direct epitaxial aggregation process in the case of ZnWO4 was clearly observed instead of the traditional Ostwald ripening process. This finding could be universal to various oxide systems or even non-oxide systems. Such spontaneous self-aggregation and crystallization induced by the well-crystallized nanorods could provide a unique route for improving the property of an anisotropic material due to their rough surface structures made of tiny nanoparticle building blocks and could be used for the formation of more complex crystalline three-dimensional structures in which the branching sites could be added as individual nanoparticles.