Abstract: Enzymatic properties of the chloroplast GAP dehydrogenase and FBPase from the wheat or the rice leaves of the main stems during grain filling period have been investigated. According to the effect of the substrate or coenzymes concentration, it is shown that the GAP dehydrogenases in freshly ruptured wheat or rice chloroplast show higher activities than FBPase, especially the activity of the wheat chloroplast GAP dehydrogenase is over ten times greater than that of FBPase. However, the activities of the above two enzymes in the rice chloroplast show only little difference, but the activity of the rice FBPase is higher than that of the wheat. The maximum initial velocities (Vmax) of the two enzymes in the above two chloroplasts are determined and are apparantly different from each other. At about 0.2 mM NAD(P)H concentration the activity of GAP dehydrogenase in the rice chloroplast is saturated. But, with the same concentration the wheat GAP dehydrogenase is still not saturated. It is noteworthy that the ratio of the activities of the two chloroplast GAP dehydrogenases for NADPH and NADH approaches equal as the coenzyme concentration is increased. Possible physiological significance of the kinetic properties of the above two enzymes during the wheat and rice grain filling period was discussed.