Abstract: An examination of 401454 new-born infants' sex data gathered from 66 medical institutions in 8 prefectures ( or municipalities) of Yunan from 1971 to 1993 reveals; 1) the new born infants, sex ratio has been increasing from 104. 06 in 1971, to 108. 57 in 1993, with the lowest year ratio of 101. 11 in 1994, the highest ratio of 111. 87 in 1991 and an average yearly increase of 0. 42; 2) the birth peaks appeared in October, November and December respectively , with the highest peak in November, and their highest sex ratio peaks scattered in the five months of March, April, June, July and September ( the highest in 109. 47 in September); 3) there is a significant different between the sex ratio in urban area and that in rural area , with the former higher than the latter; 4) the sex ratio in and before 1979 varies between 101. 11 and 105. 67, and that in and after 1980 varies between 104. 11 and 111. 87; 5) the author puts forward six strategic measures for keeping the sex ratio balance.

Key words: Sex ratio, Physical anthropology , Chinese, Yunnan