Different bagasse fibers were obtained by chemical treatment with sodium hydroxide solution. Thermal decomposition behavior of raw and treated bagasse fibers in the nitrogen gas was studied by a thermogravimetric analyzer. The thermal decomposition kinetics of bagasse fibers was modeled with three independent first-order reactions of the thermal decomposition of lignin, cellulose, and hemicellulose, respectively. The kinetic parameters and component-content constants were determined by means of the Matlab program using the least-square method. The calculated results were consistent with experimental data very well. Contents of lignin, cellulose, and hemicellulose in the bagasse fibers were calculated. The calculated contents of major raw bagasse components were close to the reported values in the literature.