Abstract: The hydrothermal carbonization (HTC) of waste cotton fibers, viscose fibers and Avicel to prepare carbon microspheres was investigated. The precursors and carbonized products were characterized by SEM, EDS, XRD, TG and FTIR. Results showed that the optimum HTC conditions for preparing carbon microspheres from cotton fibers and Avicel with high crystallinities of 60.35 and 60.24%, respectively, were respectively 330 ℃ for 6 h in 0.15% CuSO₄ and at 310 ℃ for 6 h in 0.10% CuSO₄, while they could be synthesized at 260 ℃ for 8 h from viscose fibers with a low crystallinity of 34.31% without any additive, indicating that cellulosic materials with a lower crystallinity have milder HTC conditions for preparing carbon microspheres. The polymerization degree of the cellulosic materials has little effect on the formation of carbon microspheres. The carbon microspheres produced from three cellulosic materials have a similar amorphous structure and abundant functional groups, while those produced from cotton fibers and Avicel have higher carbon contents and better thermal stability. The cellulosic materials were first hydrolyzed to form water soluble products, which underwent dehydration, polymerization, polycondensation and aromatization to form the carbon microspheres. CuSO₄ promotes the hydrolysis of the cellulosic materials which is why it is needed to form carbon microspheres from cellulose of a high crystallinity.