Spherical perovskite-type bismuth ferritic nanocrystals with diameters of 10–50 nm and hexagonal-shaped sillenite-type ones with sizes of 18–33 nm were synthesized at low temperatures by microwave hydrothermal process. Their structural, optical, and photocatalytic properties were investigated. The single-crystalline nature of the as-synthesized nanocrystals was confirmed by X-ray diffraction, selected area electron diffraction, and high-resolution transmission electron microscopy. X-ray photoelectron spectroscopy investigations show that Fe element exists as the Fe3+ valence state, as well as Bi element as Bi3+ in the as-prepared bismuth ferritic nanocrystals. The visible-light photocatalytic activities evaluated by the degradation of rhodamine B in aqueous solution show that the sillenite-type bismuth ferritic nanocrystals exhibit higher photocatalytic ability than the perovskite-type ones, which can be ascribed to their small mean particle size and the unique hexagonal-shape morphology, and also the structural characteristics of sillenite-type compound. The present results demonstrate that the hexagonal-shaped sillenite-type bismuth ferritic nanocrystals can be used as novel visible-light-responsive photocatalysts for degradation of organic compounds.