Abstract: In this study, stomatal parameters (density and index) are obtained from the same areas, the middle part of leaves, so that CO₂ and other factors have similar influences on stomatal parameters. The coefficient of variation of stomatal index is more stable than that of stomatal density in both Sequoia sempervirens and Metasequoia glyptostroboides. The results show that the stomatal index is a more useful guide than stomatal density for indicating atmospheric CO₂ concentration based on the stomatal parameters of S. sempervirens and M. glyptostroboides. The stomatal parameters vary with changes in many factors, so it is vital to reduce these effects to a minimum while reading a CO₂ signal from leafy fossils. The stomatal parameters are affected by environmental factors and stomatal density is more sensitive to these, so that the stomatal index should provide more accurate CO₂ estimates. Stomatal parameters are also related to plant species, leaf ontogeny, leaf locations of the plant and intra-leaf areas.