Abstract: Carboxylation rate of Rubisco (RCR), activity of carbonic anhydrase (CA) and intercellular CO₂ concentration (Ci) were analyzed in September and December, 1998, in leaves of forest plants Schima superba, Castanopsis fissa, Psychotria rubra and Ardsia quinquegona seedlings grown at three different light intensities, which were 100%, 36% and 16% of natural sun light. RCRand CAactivity of all tested plants were decreased obviously by declining growth light intensity, In comparison with plant at 100% sunlight, in September, the RCRand CAactivity in leaves of Schima superba grown at 16% sunlight declined by 55% and 50%, respectively, while that in leaves of Castanopsis fissa declined by 20% and 35%. The decreasing ranges of RCRand CAactivity in shade adapted shrub species. Psychotria rubra and Ardisia quinquegona were less than that of tree species, which decreased by 33%～38% in RCRand by 22%～30% in CAactivity. In the same case, the levels of RCR and CA activity measured in December were lower than that in September. Similar changing trend of RCRand CAactivity was also found in plants grown in different sites of natural forest with different light conditions sampling in January, next year. There was a positive relationship between RCR and CA ac tivity , and a weak negative relationship between Ci and RCRor CAactivity. It is proposed that higher irradiance might promote the activation of Rubisco and induce the increase in CAactivity and transport rate of DIC (dissoluble inorganic carbon).