Microcrystalline cellulose (M.C.C.) having various ratios of crystal region to amorphous one obtained by grinding. Crystallinity was determined for M.C.C. and ground M.C.C. by X ray diffraction method (powder method) using Hermans'theory. Crystallinity was also determined by infrared measurement in converting hydroxyl groups into deuteroxyl groups in amorphous region of M.C.C. Crystallinities by both methods showed linear relationship for the samples. Specific surface area was measured by adsorption methods of nitrogen gas and water vapor for M.C.C. and ground samples. The same value of 1 m2/g was given by nitrogen gas adsorption for M.C.C. and ground samples. In water vapor adsorption ground samples increased in surface area with grinding time from 150 m2/g of intact M.C.C. to 360 m2/g of 32 hr ground M.C.C. This is due to the conversion of crystal into amorphous state by mechanical force. The linear relationships between specific surface area by water vapor adsorption and heat of wetting in water was obtained for ground samples. Porosity in compression and hardness of tablet was measured for ground samples. It was found that intact M.C.C. showed larger porosity in compression and higher tablet hardness than ground M.C.C.