The thermal distribution of polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) with sizes close to the nanoregime is analyzed by means of a fluorescent nanothermographic imaging technique based on the heat-induced fluorescence feature of polydiacetylene (PDA) supramolecules. The direct detection of nanoscale hot spots generated by hot carriers in poly-Si TFTs is demonstrated with sufficient spatial resolution. The thermal information of poly-Si TFTs under operation is recorded in a PDA-embedded polyvinyl alcohol film in the fluorescence state. The proposed thermal analysis method for poly-Si TFTs overcomes the fundamental spatial resolution limitation of conventional infrared detection systems and guarantees nanoscale spatial resolution. This approach, which offers cost effectiveness, nontoxicity, and simplicity of calibration steps, can be useful for further analysis of the degradation mechanism and reliability issues of submicrometer poly-Si TFTs.