Terrestrial plants are important indicators of palaeoclimate and palaeoenvironment during the geological history. The Ginkgoales leaves are commonly used to reconstruct the atmospheric palaeo-CO2 concentration (pCO2) during the Jurassic. Baiera, a genus of Ginkgoales, however, has been less reported in the reconstruction of the pCO2 during the Middle Jurassic. In this paper, Baiera hallei Sze from the lower Middle Jurassic (Aalenian) Yan'an Formation in the central Ordos Basin is studied in detail based on abundant materials. Ten well-preserved leaves of B. hallei were selected to calculate stomatal parameters, and their stomatal index was used to estimate the pCO2. The average SI was 6.19 +/- 0.38, the pCO2 obtained by the stomatal ratio method was 1121 +/- 73 ppm, and the average palaeotemperature reconstructed by McElwain's empirical formula was 13-15.8 degrees C. The pCO2 roughly coincides with the long-term carbon cycle model GEOCARB II. The palaeoclimate in Yulin district was warm and humid during the Aalenian (early Middle Jurassic). (c) 2025 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.