Ribulose-1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) is one of the enzymes that exist most abundantly on the earth. It catalyzes a carbon dioxide fixation reaction in photosynthesis. It is of importance to increase the activity of RuBisCO and to reduce carbon dioxide for solving lack of natural resources and recent environmental problems. The 31P-NMR analysis of the carbon dioxide fixation reactions catalyzed by RuBisCO have been investigated by using sub- or supercritical carbon dioxide as a substrate. The reaction proceeded very rapidly under these conditions as ompared with normal conditions and the reaction rate was extremely improved up to 36times.