We have found that poly(benzobisimidazobenzophenanthroline) (BBL) nanobelts can be prepared by a simple high-yield, solution-phase process, which enables dispersions of the nanobelts in a large number of solvents including environmentally benign solvents such as methanol and water. Characterization of the nanobelts by transmission electron and atomic force microscopies, electron diffraction, and X-ray diffraction showed that the BBL polymer chains are oriented parallel to the long axis of each nanobelt. This unique packing motif is unlike the reported packing of polymer chains in other nanostructures, such as poly(3-hexylthiophene) nanowires, where the polymer backbone packs face-to-face along the nanowire direction. This unusual molecular packing in BBL nanobelts is explained by the rather strong intermolecular interactions, which are a result of the rigid and planar polymer chains. We investigated electron transport in single nanobelts and nanobelt networks via field-effect transistors and observed mobilities up to ∼7 × 10^-3 cm² V^-1 s^-1 and on/off current ratios of ∼1 × 10⁴. The n-channel nanobelt transistors showed stability and repeatability in air for more than 6 months, which is the most stable among current n-channel polymer transistors. These results demonstrate that the BBL nanobelts are promising for organic electronics and nanoelectronics.