Abstract: Generally,gibbons are monogamous and territorial,mean group size is about four.But with the disputable species of black-crested gibbon (Hylobates concolor) based on recent ecological studies,the mean group size vairied from 3 to 7 or 8 according to different studies,and the mating system was considered to be monogamous or polygynous (Xu et al.,1983;Haimoff et al.,1986;Liu,1988;Lan,1989).This article mainly deals with the black-gibbons group size and composition,after a 10-month survey of black-crested gibbons in Wuliang and Ailao Reserve,Yunnan Province.We concluded from seven groups in 4 sites that mean group size is 4.3,each group has 1 adult male,1-2 adult females,and 1-3 offspring of various ages.The group size and composition is similar with that of Hainan black-crested gibbon reported by Xu et al(1983) and Liu (1988),but the results show some differences with Haimoff et al (1986) and Lan (1989).The group size and compositioon are quite correlated with the habitat,e.g.,GC1 group and GD group live in the less disturbed area,each maintians a group of six or seven individuals including two adult females respectively,and GG group only has one adult female and one subadult male because of heavy hunting presure and forest cutting in their habitat,althouth they live in more than 200 hm2.