White-cheeked Gibbon (Nomascus leucogenys Ogiby, 1840) are rare primate species: EN in Red Book of Vietnam, 2007, Decree No.32CP/2006-group IB, group I in CITES-11/2002/ND- CP and EN in the IUCN Red List (2011). The number of individuals is now seriously reduced due to habitat loss and hunting. There is lack of information on preferred habitats and distribution, as well as threats to species and habitats in the reserve. Therefore, the authors conducted a study for the state of White-cheeked gibbon (Nomascus leucogenys) in Xuan Lien Nature Reserve, Thanh Hoa province. The findings include 41 groups with an estimation of 127 individuals of White-cheeked gibbon in Xuan Lien Nature Reserve. The gibbons are distributed in many different habitats, but is most concentrated in two forest habitats and IIIB IIIA3 located in the southwest and northwest of Xuan Lien Nature Reserve, adjacent to the Pu Hoat Nature Reserve, Nghe An provice. Average density throughout the study area is 0.64. IIIB forest contains highest density (1.1B and 3.85 group/km2 mixed forest state has the lowest density of gibbons (0.08 group/km2 and 0, 31 group/km2) .