Nitrogen gas adsorption isotherms are obtained on intact, 8 and 32 hours ground microcrystalline cellulose. Before the determination of adsorption isotherms, each specimen is dispersed into water, then the water is changed to ethanol, ethanol to n-pentane, and the n-pentane is evaporated in vacuo. Specific surface area is determined from the nitrogen gas adsorption method as 79, 153 and 250 m²/g for each sample, using B.E.T. equation. This result shows that the effective surface available to water molecules is equivalent to that available to nitrogen molecules. Pore size distribution was determined from both nitrogen gas desorption isotherms and X-ray small angle scattering. Significant change in pore size distribution by grinding is not observed. The effective surface area of microcrystalline cellulose is composed of pores which range in diameter is up to 300 Å, and relatively small pores, up to 100 Å, play an important role. Recrystallization of amorphous cellulose is observed to be caused by the solvent displacement, to the same crystal form as cellulose II. The possible relation of these observations to the fine structure of ground microcrystalline cellulose is discussed.