Abstract：Animal’s immune functions are not only closely associated with environmental conditions but also may play a role in regulating population dynamics. The Qinghai-Tibet Plateau has a severe and long cold season (September - April) and short warm season (May - August). To explore seasonal variations in immune function of root voles (Microtus oeconomus), we captured root voles monthly by using live-trap methods from May 2006 to April 2007 at the Haibei Research Station of Alpine Meadow Ecosystem,the Chinese Academy of Science,and monitored spleen mass and delay hypersensitivity induced by dinitrofluorobenzene. To examine the variation of immune parameters in the cold season,it was divided into cold season 1 (September -october,2006),cold season 2 (November,2006 - January,2007)and cold season 3 (February - April,2007). Individual’s spleen mass and delay hypersensitivity response in cold season 1 did not significantly differ from those in warm season,but the spleen index in cold season 2 and cold season 3 was significantly reduced compared to cold season 1 and warm season,respectively. Individual’s delay hypersensitivity response in cold season 3 was also significantly lower than those in cold season 1 and warm season,respectively. Age differences in spleen mass were found during the cold season 2,with sub-adults having smaller spleen weights compare to adults. The results show that immune function of root voles is gradually reduced during the cold season. We suggest that low temperature in Qinghai-Tibet Plateau may be responsible for the seasonal variation in immune parameters. Our results also suggest that the age difference in immune function may play an important role in population auto-regulation for root voles.