Abstract: To analyze the diet and dietary composition of eastern hoolock gibbons (Hoolock leuconedys), to understand how their diet and dietary composition changes seasonally and annually, and to probe the strategies and adapting mechanism of eastern hoolock gibbons reply to the food changes, we conducted field observation at Nankang, Mt. Gaoligong, Yunnan, China. The feeding habits and behaviors of eastern hoolock gibbons in the spring and the autumn of 2007 and 2008 were observed and recorded using focal animal sampling method. Feeding behaviors of eastern hoolock gibbons were divided into eating leaves, fruits, worms, flowers and stems, and drinking fruit juice. Their dietary composition was divided into fruits, young leaves, flowers, stems and animal matters. Meanwhile, the feeding amount and percentage of each vegetative food source and dietary composition were compared and calculated. The results showed that eastern hoolock gibbons had 16 vegetative food sources in spring and 23 in autumn. Four sources in spring and 5 in autumn accounted for a relatively high proportion among all food sources. The ShannonWiener index of the vegetative food sources in autumn was higher than that in spring and 5 vegetative food sources overlapped in springs and autumns between 2007 and 2008. The result of Crosstabs χ2 test showed that the feeding amount of the 5 overlapping vegetative food sources was strikingly different when the same season was compared. Although the results showed that ShannonWiener index of the types of the vegetative dietary composition in spring was higher than that in autumn, yet the dietary compositions in spring and autumn were similar, i.e. fruits accounted for the majority, followed by young leaves, flowers and stems in turn; animal food were hunted in both spring and autumn, but this phenomenon only occurred occasionally. The fact that diet of eastern hoolock gibbons at Nankang, Mt. Gaoligong varied from season to season could be regarded as their adaptation to the changes of habitat conditions. One of their vital feeding strategies was to keep the dietary composition relatively stable annually and seasonally.