Objective To investigate the role of H3K27me3 demethylase Jmjd3(KDM6B) during the development of lung in embryonal mice .Methods Jmjd3 knockout embryos of E 19.5 mice were examined by HE , PAS and immnohistochemistry assays .Results The developmental defects of the lung of Jmjd 3 heterozygous ( Jmjd3 +/-) embryos were mild is compared to Jmjd3 +/+embryos.However, Jmjd3 -/-mice suffered from the severe hypoplasia of lung tissue .Differentiated defects of ciliated cell , Clara cell , type Ⅰ and Ⅱ alveolar epithelial cells were ob-served in Jmjd3 -/-embryos.The index of cell proliferation was increased in Jmjd3 -/-embryos as compared to wildtype and Jmjd3 +/- embryos.No difference in apoptosis profile was found in these embryos .Conclusions Jmjd3 is essential for proliferation and differentiation of embryonal lung epithelia of mice .