The synthesis of high-quality tetragonal Mn3O4 nanoparticles and the large-scale assembly of the nanoparticles into highly regularly walls using a simple solution method at low temperature and surfactant-free conditions was carried on. This synthesis route is easily controllable, well-repeatable, mild, and feasible for application to the fabrication of nanopatterns of other materials. Transmission electron microscopy (TEM) and the corresponding selected-area electron diffraction (SAED) pattern show uniform Mn3O4 tetragonal nanoparticles. The sharp SAED pattern, which can be obtained from any of the nanoparticles within the nanopatterns and parallel lattice fringes in high-resolution TEM (HRTEM) images, indicates that all of the quantum dots are Mn3O4 single crystals aligned along their c axis. The HRTEM images also show a finely dispersed configuration of the nanopatterns.