In this study, the effects of particle size on combustion kinetics, mechanisms, and decomposition characteristics of five wood samples were examined using thermogravimetric analysis (TGA) under air atmosphere and three different heating rates (10, 40, and 80 °C/min). By using the both Flynn-Wall-Ozawa and Coats-Redfern methods, the highest activation energies were found to be for the coarse samples except untreated pine samples. Only a few studies in the literature investigated the effect of particle size on both thermal kinetics and responsible mechanisms of solid biomass fuels under oxidative environment. Therefore, the findings can be taken into consideration for highly efficient design and operation of industrial thermochemical processes by using waste lignocellulosic biomass as a fuel. In this study, the average activation energies calculated by Flynn-Wall-Ozawa were 166.57, 153.08, and 171.42 kJ/mol for the smallest, medium and coarse samples, respectively. A similar trend but lower values were also evaluated by Coats-Redfern and it was stated as 131.34, 137.52, 146.22 kJ/mol for the smallest, medium, and coarse samples, respectively. As particle size increased lower combustion performance indexes were also obtained. Therefore, in addition to the particle size, the chemical content of the samples also had an impact on the alteration of the kinetic mechanisms and thermal decomposition properties of the samples.