The thermal degradation behavior of different types of cellulose before and after mechanical defibrillation and lyophilization was studied using isothermal and nonisothermal thermogravimetric analyses, followed by other characterization techniques, such as X-ray diffraction, Fourier transform infrared spectroscopy, degree of polymerization and scanning electron microscopy with field emission analysis. The thermogravimetric experiments were carried out in a nitrogen atmosphere at four different heating rates (5, 10, 20 and 40°C min -1 ) in a nonisothermal condition. Distinct thermal degradation behaviors were observed when the two types of cellulose were compared after defibrillation: (1) cellulose nanofibers tend to lose thermal stability and (2) cellulose nanowhiskers tend to gain thermal stability. The Flynn-Wall-Ozawa method results indicate that the apparent activation energies calculated for the cellulose fiber sample has higher values requiring more energy for the thermal decomposition. Criado curves indicated a degradation mechanism for the cellulose: one-dimensional diffusion.