Abstract: Carbon microspheres(CMs) were prepared by the carbonization of waste cotton fibers in subcritical water, and were characterized by SEM, XPS, XRD, FTIR and elemental analysis. The amount of fibers as a function of volume of water, carbonization temperature and time was optimized based on the morphology, elemental composition and size of the CMs. Results indicate that the best CMs have the highest fraction of spheres with sizes from 0.8 to 3μm and a carbon content of 74.99 wt% and are obtained under subcritical water at 280℃ for 10 h when the amount of cotton fibers is 20 g per liter of water. The CMs have an amorphous structure and their surface C/O ratio is higher than the global C/O ratio. The cotton fibersare converted into CMs by hydrolysis, cracking, polymerization, condensation, aromatization and finally spheroidizing to decrease surface energy.