The present paper describes the study of the effect of mechanochemical activation on the amorphization process of individual α-cellulose and native cellulose being a constituent part of the lignocellulosic material in the form of partially crystalline fibrils. In processing the powder X-ray diffraction data the following methods are used to determine the degree of crystallinity of cellulose: Segal’s, Rietveld’s, and Lorentzian deconvolution. It is demonstrated that mechanical activation of individual α-cellulose in an AGO-2 laboratory planetary ball mill with a shock-shear action results only in grinding and amorphization, while the degree of amorphization increases propotionally to the duration of the power supply. When α-cellulose is treated in an RM-20 flow-through centrifugal roller mill with a shear action, particle agglomeration is observed together with amorphization. When a lignocellulosic material (wheat straw) is treated in a centrifugal roller mill, considerable amorphization occurs only at high energies, and no particle agglomeration is observed.