Thermo-gravimetric analyses of three cellulosic substances, namely, microcrystalline and amorphous cellulose, and treated Japanese cypress (JC) sawdust were carried out in this study. The thermal degradation temperature of crystalline cellulose decreased with increasing ball-milling time, while that of amorphous cellulose barely changed. However, small differences in the derivative thermo-gravimetric (DTG) curves between crystalline cellulose (i.e., before ball milling) and amorphous cellulose (i.e., after ball milling) were observed. The DTG curves of high-crystalline cellulose were sharp and similar to those of low-crystalline samples. The thermal degradation temperature of JC was decreased by ball milling, and its DTG peak shape became broad and low. These effects could be caused by the denaturing of non-cellulosic substances such as hemicellulose and lignin. The thermal degradation behaviors revealed by the DTG curves may serve as indicators of crystalline cellulose purity and other physical properties of lignocellulosic biomass.