Abstract: The primary food for blackandwhite snubnose monkey (Rhinopithecus bieti) including lichen (Tillandsia usuneoides), mature tree leaves and bamboo leaves were sampled in November 2005 at Mt. Longma where it was the most southern distribution area for this animal, and the nutrition composition of these food samples were measured in doors. The results showed that the crude protein content of lichen was lower than that of bamboo leaves, but there was no difference in the crude protein content between the lichen and mature tree leaves. The ratio of crude protein and acid deterrent fiber (CP/ADF) in the lichens was higher than that of in the mature leaves and in bamboo leaves. This difference in the CP/ADF ratio was mainly due to the difference in the acid fiber content among the three types of food, i.e., the acid deterrent fiber content of the lichens was significantly lower than either mature tree leaves or bamboo leaves. It was preliminarily determined that the lower fiber content was one of the factors for blackandwhite snubnose monkey to choose the lichens as its primary food. This finding was consistent with the fact that blackandwhite snubnose monkey tended to choose plant leaves with low fiber content as food.