Thermogravimetric (TGA) analysis was used to study the thermal decomposition and oxidation, combustion characteristics, and kinetics of cotton stalks and sugarcane bagasse powder of various particle sizes under air from room temperature to 1000 oC at heating rates 25, 50, and 75 oC /min. It was observed that all the samples followed a two-stage reaction mechanism between 200 and 1000 oC clearly indicating regions of volatile oxidation and char combustion during thermal decomposition and oxidation. It was also found that heating rate, particle size, lignocellulosic composition, and gas flow types affect the thermal decomposition mechanism and kinetic parameter values of both materials. The kinetic parameters were determined using simple Distributed Activation Energy Model (DAEM) and three methods of model-free kinetics. It was found that average activation energy (Eav) for cotton stalks and bagasse for both bulk and different particle sizes ranged between 34–118 kJ/mol and 87–187 kJ/mol, respectively. Results also showed that the values of kinetic parameters obtained from all methods are in a good agreement and can be successfully used to understand the degradation mechanism of solid-state reaction of these biomass materials. Finally, a comparison between the thermal pyrolysis characteristics of both materials under inert (nitrogen) and air at a heating rate of 10 oC /min is presented and discussed.