In this work, the thermal degradation behaviors of two kinds of biomasses (pinewood and rice husk) with powder and pellet under three oxygen concentrations were investigated by a self-designed macro-thermogravimetric analyzer. An obvious hysteresis of thermal degradation of biomass pellets was observed under three conditions. The maximum activation energy of biomass pellets was significantly greater than that of biomass powders, while their average activation energies were almost equal based on distributed activation energy model. For the oxygen-rich combustion, the comprehensive combustion character index of powdered and pelletized biomasses ranged from 3.92 × 10 to 5.16 × 10%·min·°C and from 1.82 × 10 to 1.91 × 10%·min·°C, respectively. Furthermore, the derived biochar of powdered biomass has a higher caloricity than that of pelletized biomass during combustion by TG-DSC analysis. The performances of thermal degradation observed by macro-thermogravimetric analyzer could factually reveal the influence of mass and heat transfer on the thermochemical conversion of powdered and pelletized biomasses.