Unusual polyhedral structures of cubic Fe3O4 were fabricated in high yield via a facile hydrothermal method in the presence of a surfactant cetyltrimethyammonium bromide (CTAB). Hexagonal, dodecahedral, truncated octahedral, and octahedral shapes can be prepared by changing the concentration of the CTAB in l-arginine solutions. In the solution-phase synthesis, both the l-arginine and CTAB were found to be the prime factors for the formation of the polyhedral structures. In addition, room-temperature vibrating sample magnetometer (VSM) testing results indicated that the magnetite synthesized without the addition of CTAB had the highest M (s) value and the lowest H (c) value. This synthesis method is also suitable for the preparation of other ferrites with spinel structure.