The selected-control synthesis of uniform α-Fe₂O₃ nanoparticles with morphologies of rhombohedra, rods, and cubes was successfully achieved in large quantities by the capping agent–cationic surfactant–CTAB-mediated hydrolysis of FeCl₃. The nanoparticles were obtained when the concentration of FeCl₃ ranged from 0.01 to 0.05 M at 120 °C. The growth mechanisms of α-Fe₂O₃ nanorhombohedra, nanorods, and nanocubes were analysed in detail, based on the topotactic transformation of the precursor β-FeOOH with different morphologies under the capping agent mediation. The concentration of FeCl₃, the reaction temperature, and the concentration of CTAB are all responsible for the final crystalline morphologies of the α-Fe₂O₃ nanoparticles. This result may facilitate not only the exploration of controlled approaches of preparing α-Fe₂O₃ nanoparticles for potential technical applications but also a deeper understanding of the fundamental physical and chemical processes of hydrothermal methods. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and field-emission scanning electron microscopy (FE-SEM) were used to characterize the products.