The characteristics of spatial and temporal distribution and climate causes of hial formation in Yunnan were studied based on the data of the number of monthly hail day(1961-1997) at 120 stations of Yunnan Province.Hail usually occurs in February,March or April in Yunnan because it belongs to typical spring hail region.The number of hail days in spring accounts for 64 percent of the total number in the whole year.There is significant interannual change of hail in spring.The severest hail year is 1990,and the lightest hail year is 1984.In view of the region the southwest Yunnan is a major region for hail occurance with the number of hail days made up 30.4 percent of the whole province,while,the middle Yunnan is the second.In addition,there exits significant differences of polar vortex intense,height field on the Qing zang Plateau,trough in India Myanmar,and OLR in Northern Hemisphere between above normal hail year and below normal hail year in spring.