Effective pi-electron delocalization within a PTCDI nanobelt, for which the one-dimensional molecular arrangement is dominated by the cofacial pi-pi stacking, has been characterized by both electron spin resonance (ESR) spectrometry and current-voltage (I-V) measurement. The long-range pi-electron delocalization enables dramatic enhancement of electrical conductivity of the nanobelt through external-charge doping. When immersed in saturated hydrazine vapor, about 3 orders of magnitude increase in conductivity was achieved for the PTCDI nanobelt, implying potential application of the nanomaterials in electrical sensing of reducing gaseous species such as organic amines.