Ge3N4 nanobelts 30-300 nm in width were synthesized by thermal reduction of a mixed Ge+SiO2 powder in NH3 atmosphere. These nanobelts were studied by high-resolution transmission electron microscope equipped with an x-ray energy dispersive spectrometer. In these synthesized nanobelts, the existence of α and β phases of Ge3N4 was identified. The a phase exhibiting slight difference from an ideal α-Ge3N4 phase was also found in the present Ge3N4 material. The mechanism of formation of the Ge3N4 nanobelts is discussed.