Ultralong nanobelts (>0.3 mm) have been fabricated from an asymmetric perylene tetracarboxylic diimide (PTCDI) molecule via a seeded self-assembly processing. The long length of nanobelts facilitates the construction of two-electrode devices employing the nanobelt as channel material, and the long-range one-dimensional π−π molecular stacking allows for efficient conductivity modulation through surface doping. A combination of these two characters enables efficient electrical sensing of reducing VOCs using the nanobelt. As examined for hydrazine, more than 3 orders of magnitude increase in current was observed for a single nanobelt when exposed to the saturated vapor of hydrazine.