Abstract: The evolution of functional groups during the preparation of cellulose-based carbon fibers (CFs) was investigated by nanoscale infrared spectroscopy (Nano IR) of the fiber cross-section and surface. The cross-sections of cellulose-based fibers at different stages of low-temperature pyrolysis from room temperature to 600℃ were obtained by an ultra-thin sectioning technique. The infrared spectra and maps of the functional groups at different locations on the cross-section and the surface of a fiber were obtained by the Nano IR. Results showed that C-O and CO groups were distributed evenly on the fiber surface, but -OH groups were unevenly distributed. The -OH group concentration was higher near the surface than in the inner area, but CO groups were distributed evenly on the fiber cross-section. The number of both -OH and CO groups started decreasing at 150℃ and levelled off at 300 and 600℃, respectively. Pyrolysis reactions proceeded from the outer to the inner part of the fiber, indicating that the skin and core were not pyrolyzed simultaneously. I_D/I_G values from Raman spectroscopy also confirmed the structural heterogeneity in the cross-section at the different pyrolysis temperatures examined. This study is useful to effectively optimize the preparation parameters of CFs with improved properties.