Quantitative reconstructions of atmospheric CO2 by using terrestrial and marine records are critical for understanding the so-called “greenhouse” conditions in the Cretaceous, but data from terrestrial plants for several stages of this period remain quite limited. Using the stomatal index (SI) technique, here we estimate the Santonian (Late Cretaceous) CO2 contents based on a sequence of fossil cuticles of Ginkgo adiantoides (Ung.) Heer from three beds of the Yong’ancun Formation in Jiayin, Heilongjiang Province, northeastern China. By the regress function, SIs of Ginkgo fossils reveal a pronounced CO2 reduction from the early to late Santonian (~661 and ~565 ppm, respectively). The relatively high CO2 levels provide additional evidence for paleoclimatic warmth in this interval. Moreover, available paleobotanical data illustrate a decline trend of CO2 contents throughout the Late Cretaceous, punctuated by several fluctuations in particular episodes with different magnitudes. The CO2 contents shifted notably in the late Cenomanian, Turonian, early Santonian, late Campanian, and probably latest Maastrichtian. Furthermore, a comprehensive study based on CO2 data shows that the global mean land surface temperature (GMLST) fluctu ated several times accordingly. The change ratios of GMLST (ΔT) increased from ~3°C in late Cenomanian to ~4.7°C in mid Turonian, and then dramatically reduced to ~2.2°C in mid Coniacian. From the Santonian onward, it appears that the tempera ture gradually decreased with a few minor fluctuations. Quantitative reconstructions of atmospheric CO2 by using terrestrial and marine records are critical for understanding the so-called “greenhouse ” conditions in the Cretaceous, but data from terrestrial plants for several stages of this period remain quite limited. Using the stomatal index (SI ) technique, here we estimate the Santonian (Late Cretaceous) CO2 contents based on a sequence of fossil cuticles of Ginkgo adiantoides (Ung.) Heer from three beds of the Yong’ancun Formation in Jiayin, Heilongjiang Province, northeastern China. By the regress function, SIs of Ginkgo fossils reveal a pronounced CO2 reduction from the early to late Santonian (~ 661 and ~ 565 ppm, respectively). The relatively high CO 2 levels provide additional evidence for paleoclimatic warmth in this interval. decline trend of CO2 contents throughout the Late Cretaceous, punctuated by several fluctuations in particular episodes with different magnitudes. The CO 2, 2 content shifted notably in the late Cenomanian, Turonian, early Santonian, late Campanian, and probably latest Maastrichtian. Furthermore, a comprehensive study based on CO2 data shows that the global mean land surface temperature (GMLST) fluctu ated several times accordingly. The change From the Santonian onward, it appears that the tempera ture may be reduced to ~ 2.2 ° C in mid Coniacian. with a few minor fluctuations.