Constructing carbon nanotubes and related segments with atomic precision remains challenging, despite significant advances, including cycloparaphenylene (CPP), carbon nanobelts, and phenine nanotubes. Here, we report three conjugated nanobelts (NB1-NB3) constructed by longitudinal π-extension of CPP with rings of the same, smaller, or larger size. Each nanobelt comprises three fused aromatic units bridging two axial CPP rings, which mediate the intermolecular assembly. NB1-NB3 have a delocalized electronic structure, showing largely red-shifted emission. The smaller NB2 can insert into NB3 to form a stable double-walled assembly (NB2⊃NB3), which exhibits energy transfer with nearly 100% efficiency.