Unique ultrathin CoSe2-DETA (protonated) mesostructured nanobelts with multiple stacked layers which are highly parallel to the axial direction have been first prepared in a binary solution composed of organic amine and water under mild solvothermal conditions. This synthesis strategy may open new avenues toward the syntheses of other new mesostructured nanomaterials, which may bring new nontrivial functionalities.