Abstract: Western Liaoning,China,is world-renowned for the Mesozoic Jehol biota,especially for yield-ing many feathered dinosaurs,primitive birds,mammals and fossil angiosperms .In autumn of 1997,the author found a complete specimen of a symmetrodont mammal with well-preserved hair and soft tissue from the basal part of the Late Jurassic to Early Cretaceous Yixian Formation in the Sihetun area,Beipiao City,western Liaoning.It is very significant for our understanding the morphology,osteology,phylogeny and life habits of Mesozoic symmetrodont mammals.