Group size and composition of blue sheep(Pseudois nayaur)were studied in the Helan Mountains, Ningxia Autonomous Region from November to December 2003.We scanned mountain slopes with binoculars and observed with 20-60× spotting-scope. A total of 310 herds of blue sheep and 1 336 individuals were observed during the study period. Blue sheep were frequently seen in small herds of 2 to 8 individuals, which represent 94.8% of total herds observed. Herds consisting of 9 individuals or more represented 5.2%. The largest herd we observed numbered 51 individuals. Mean group size was 4.2 individuals. Blue sheep herds can be divided into three types: male herds (composed solely of males), female herds (consisting of females with or without juveniles of both sexes), and mixed herds (including adult males, females, and subadults).Among the 310 herds, female herds were counted 150 times(48.4%), mixed herds 154 times(49.7%), and male herds 6 times(1.9%). Of 1 336 blue sheep classified by sex and age, adults, subadults and juveniles composed 64.1%,20.8%, and 15.1% respectively. The female:male ratio of adults was 1∶0.73. The ratio of adult females to juveniles was 1∶0.56, which is higher than the ratio recorded in spring (1∶0.43) or summer (1∶0.44).The results showed that the Helan Mountains State Nature Reserve has succeed in protecting blue sheep.