Abstract: The Xinmin Formation in Zhamuqin area of Horqin Right Wing Middle Banner of Inner Mongolia were redefined by Inner Mongolia 1:50000 regional survey of four sheets such as Guonaotewula. According to lithologic combination, Xinmin Formation is divided into two lithogic sections:intermediate-acid pyroclastic rock is dominated by a small amount of pyroclastic rocks formed in lower segment overlying Linxi Formation with angular unconformity, whereas the upper part is a thick and thin clastic assemblage, which contains Pinuspollenites, Abiespollenites, Perinopollenites and some other Early-Middle Jurassic micropaleontologic fossils, with no top observed. LA-ICP-MS zircon U-Pb ages are 162.1±1.3Ma and 154.4±2.3Ma, suggesting Middle Jurassic.