Abstract: Having analysed the sex ratio (male:female 100) data based on the census of 1990 in Yunnan Province, we got the following results:
1) The sex ratio (105.67) of the total population in Yunnan is 2.09 higher than that of 1982, 0.93 lower than that of the whole country (106.60), and 1.8‰ higher than the "comparatively balanced ratio" (1003.5:1000).
2) The sex ratio of the Jingpo is especially low (95.77), and that of its age group of 0-14 years (101.81) is also the lowest among all ethnic groups; and that of the Zang or Tibetan is comparatively low (98.82), its sex ratio of the group aged 0- 14 is 105.75. The other ethnic groups with low sex ratio are: the Dai (100.34), whose, sex ratio in the group aged 0-14 is 101.99; the Naxi (101.63), that for its group at the age of 0-14 is 103.00; the Hui (101.99), with a sex ratio of 104.02 in the group of 0- 14 years; the Bai (102.05), whose sex ratio of the group aged 0-14 is 102.47; while the Han nationality has a sex ratio of 107.47 for the group of 0-14 years in age.
3) Those who born during the period of the "Great Leap Forward' and the "Temporal Difficulties in National Economy", at the age of 20-24 at the national census in 1982, have a especially low sex ratio of 101.11. However, they have a sex ratio of 107.84 by 1990's national census (now at the age of 28-32). The former is mainly due to the increased spontaneous abortion and still birth, while the latter is caused by the floating population.
4) The sex ratio in Yunnan Province has been increasing constantly. The average annual increased rate in the eight years after 1982 is 2.19 times of that of the eighteen years back from 1982.
5) The sex ratio of the urban inhabitants (113.01) is apparently higher than that of the rural ones (105.67), but the difference is being decreased.
6) The sex ratio varies in different areas in Yunnan Province. Three prefectures, Yuxi, Dali and Dehong; are minority districts with low sex ratio. But their sex ratios are increasing quite quickly, that is, 2.17, 2.11 and 2.69, respectively, higher than those in 1982.

Key words: Sex ratio; Physical anthropology