Abstract: Twenty-six named species of 23 genera have been known in China from the deposits ranging from the Early Jurassic to Late Cretaceous. Except a few taxa, most of recently discovered Mesozoic mammals in China are represented by well-preserved skulls and skeletons. They have greatly improved our knowledge of the relationships of the major mammalian lineages and the character transition during the evolution of early mammals. Some localities of the Jehol Biota produced most Mesozoic mammal fossils in the past several years. These fossils represent different major groups and show high diversity in body size, locomotory adaptation, and dietary. Such diversification ensures them to occupy different niches in the common ecosystem. Frequent volcanic eruptions in western Liaoning were primarily responsible for the massive death of animals. Specimens preserved in sleeping posture, like the type specimen of Repenomamus gz.ganticus, may indicate that the emission of poisonous gas with the volcanic eruption was probably involved in causing the disaster.