Recently, one-dimensional nanostructures have attracted extensive attention since they are potentially important for both applications and fundamental research. In this paper, we report the synthesis of ultra-long single crystal Si3N4 nanobelts via catalyst-assisted pyrolysis of polymeric precursors. The obtained products contain both alpha- and beta-Si3N4 nanobelts, which are 50-100 urn in thickness, 400-1000 urn in width, and a few hundreds of micrometers to several millimeters in length. Different from previous techniques for synthesizing one-dimensional structures, the current nanobelts are synthesized through confined crystallization of an amorphous phase. A solid-liquid-gas-solid reaction/crystallization growth mechanism is proposed. The formation of nanobelts instead of nanowires is attributed to the anisotropy growth at an earlier stage.