On the view point of the submicroscopic cellulose structure, the mechanism of the cellulose reaction has been mainly investigated by means of the x-ray diffraction method. This method is not adequate to detect the accessible property of cellulose during the reaction, but can detect the thoroughly constructed crystalline structure. Such an accessible property is related with the area of the crystalline surface, the fine crystallite, the fringe of micell, the type of hydrogen bond, and the quantity or property of the substituents. And yet, the reactivity of cellulose is rather close to the accessible property than the crystalline structure of cellulose, and also, the difficulty of separating x-ray interferences which are originated from the unreacted and reacted crystallite is frequently encounted. Therefore, the satisfactory interpretation on the mechanism of the cellulose reaction cannot be built up by the only x-ray diffraction method.On this report, the new method which involves the infrared spectrum examination accompanied by the deutration of the partially reacted cellulose is devised, and the mechanism of the nitration is investigated by this method (infrared method) along with the x-ray diffraction method. The results are shown as follows:As the nitration by the HNO3-H3PO4 mixture, proceeds, -1) accessibility of the hydroxyl group increases.2) the spectrum figure of the stretching mode of the unaccessible hydroxyl group differs from the unreacted cellulose, and becomes broad, 3) the spectrum of OD stretching mode which results from deutration of the accessible hydroxyl group has two peaks.From these phenomena, it becomes clear that a large quantity of the unaccessible hydroxyl group convertes into accessible, and also, the type of hydrogen bond of the unaccessible hydroxyl group changes as the nitration proceeds. Therefore, the mechanism of nitration by the HNO3-H3PO4 mixture may be considered as follows; that the reaction-reagent penetrates into the crystallite, and most of hydroxyl groups on the cellulose chains are uniformly nitrated.On the case of the nitration by the HNO3-H2SO4 mixture, 1) at the initial period, the increase of the accessibility of the hydroxyl group is not greater than the case of the HNO3-H3PO4 mixture, the spectrum figure of the unaccessible hydroxyl group is similar to the unreacted cellulose, From these findings the existence of region which is not concerned in the reaction is suggested.2) at the later stage of nitration, the spectrum becomes similar to that of the HNO3-H3PO4 mixture. From these results it is considered that at the initial period of the nitration on the HNO3-H2SO4 mixture the reaction has a considerable non-uniformity, but convertes into uniformity with the proceeding of the nitration.The results obtained by infrared method have good correlativity with the results of the x-ray method.