Magnetite nanoparticles of Fe3O4 have been found to grow into large highly branched nanostructures including nanochains and highly branched nanotrees in the solid state through a postannealing process. By varying the preparation conditions such as annealing time and temperature, the nanostructures could be easily manipulated. Changing the starting concentration of the magnetic nanoparticle solution also caused significant changes of the nanoarchitectures. When the magnetic nanoparticle concentration is low, the nanoparticles formed straight rods mainly with an average diameter of 80 nm and a length of several microns. With increasing concentration of the nanoparticles, treelike structures began to form. With further increase of the concentration, well-ordered nanostructures with the appearance of snowflakes were generated. The driving force for the formation of the highly ordered nanostructures includes interaction between the nanoparticles and interaction through surface-capping molecules. This experiment demonstrates that novel nanostructures can be generated by self-assembly of magnetic nanoparticles under the solid state.