A facile and environmentally friendly hydrothermal approach has been employed to synthesize size-controllable quasi-cubic α-Fe2O3 nanocrystals by using an aqueous solution of FeCl3 without any alkaline materials and stabilizing agent. The as-prepared products were carefully characterized and the growth mechanism is also discussed. This hydrothermal synthesis of such quasi-cubic structures implies a simple and low-cost route to prepare monodisperse nanomaterials on a large scale. The size-controllable synthesis of α-Fe2O3 is the first achieved successfully from 80 nm to 2 μm with constant structures. The synthesized α-Fe2O3 materials have different optical properties and stabilities in water, which are caused by the change of their band-gap energy and specific surface areas. Finally, the dielectric properties of synthesized α-Fe2O3 cubes were also investigated, and the differences caused by the particle sizes are discussed.