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<p class="p">Atmospheric <i>p</i>CO₂ concentrations have been estimated for intervals of the Eocene using various models and proxy information. Here we reconstruct late middle Eocene (42.0–38.5 Ma) <i>p</i>CO₂ based on the fossil leaves of <i>Nageia maomingensis</i> Jin et Liu collected from the Maoming Basin, Guangdong Province, China. We first determine relationships between atmospheric <i>p</i>CO₂ concentrations, stomatal density (SD) and stomatal index (SI) using “modern” leaves of <i>N. motleyi</i> (Parl.) De Laub, the nearest living species to the Eocene fossils. This work indicates that the SD inversely responds to <i>p</i>CO₂, while SI has almost no relationship with <i>p</i>CO₂. Eocene <i>p</i>CO₂ concentrations can be reconstructed based on a regression approach and the stomatal ratio method by using the SD. The first approach gives a <i>p</i>CO₂ of 351.9 ± 6.6 ppmv, whereas the one based on stomatal ratio gives a <i>p</i>CO₂ of 537.5 ± 56.5 ppmv. Here, we explored the potential of <i>N. maomingensis</i> in <i>p</i>CO₂ reconstruction and obtained different results according to different methods, providing a new insight for the reconstruction of paleoclimate and paleoenvironment in conifers.</p>