Sheet-like Bi2Fe4O9 nanoparticles were Successfully synthesized by a rather facile hydrothermal process. Transmission electron microscopic (TEM) and selected area electron diffraction (SAED) studies show that the particles are sheet-like, about 30nm in the direction of thickness. X-ray diffraction (XRD) analysis indicated that the as-prepared particles were pure Bi2Fe4O9, but after storing at room temperature for half a year, gamma-Fe2O3 appeared. The magnetic properties of the nanoparticles before and after storing were characterized by a vibration sample magnetometer (VSM).