Leaf fossils from the Oligocene Ningming Formation of China were used to reconstruct palaeo-CO2 concentration. A mechanistic-theoretical model and the stomatal ratio method were used in this study. Two angiosperms and one gymnosperm with well-preserved cuticles were considered. The results suggest that CO2 was between 255 and 972 ppmv during the development of the Oligocene Ningming Formation.