Abstract:  New materials of Atlantoxerus from the Middle Miocene Halamagai Formation, northern Junggar Basin, Xinjiang, are described and studied in detail. They are assigned to two species, Atlantoxerus junggarensis Wu, 1988 and A. xiyuensis sp. nov. By studying the new specimens and old materials, diagnosis of A. junggarensis is amended as: larger size, unilateral hypsodont cheek teeth, P4 larger than M1 and M2, protoconule very weak or absent, metaconule significantly larger than metacone, mesostyle and mesostylidusually present, no mesoconid, metalophid ending before metaconid, ectolophid and entolophid developed. The new species, Atlantoxerus xiyuensis, is characterized by small size, low crowned cheek teeth, P4 being larger or equal to M1/2 in size, well-developed hypocone, weak or absent protoconule, metaconule being larger than metacone and isolated from posteroloph, usually no mesostyle; m3 without anterior cingulum, anteroconid, mesoconid, mesostylid and hypoconulid, strong metalophid extending from protoconid to the talonid basin, metalophid not connected to metaconid, and well-developed entoconid and entolophid.  Referring to their associated mammals and relevant palynological records, Atlantoxerus likely lived in warm and humid environments. However, temperature is supposed to control the distribution and evolution of Atlantoxerus more significantly than humidity. Distribution of Atlantoxerus seems to be strongly affected by global climate changes. Under the influence of the Late Cenozoic global cooling, the distribution and species diversity reduced significantly.