Population size, group composition and feeding ecology of the endemic and endangered Black-and-White Colobus Monkey (Colobus guereza gallarum) was investigated in Harenna Forest, South East Ethiopia by using the line-transect method in 10 selected blocks. During the dry and wet seasons, a mean number of 212 and 246 C. g. gallarum individuals was recorded, respectively. Also, the number of groups of the animal was higher in wet (36) than dry (31) season, but statistically insignificant (P= 0.447). Out of the recorded C. g. gallarum, 25.98% were adult males; 32.53% adult females; 14.85% sub-adult male, 11.14% sub-adult female and 15.50% were juveniles/young. Young leaves were the largest (35.1%) contributor to the diet of guereza followed by shoots (22.1%) in both seasons. Other common items were included mature leaves (20.6 %), bark (17.7%), fruit (2.8%) and flower (1.7%).  C. g. gallarum were observed feeding on 19 plant species belonging to 12 families. The highest contribution of the diet was from the family Rosaceae (41.67%), while family Fabaceae, Moraceae and Myrsinaceae were contributing 50% of the diet. Schefflera volkensii was the most consumed plant species which accounted for 7.96%,  followed by Urera hypselodendron (7.83%)  and  Dombeya torrida (7.06%). Moreover, C.g.gallarum mainly forage on leaves particularly young leaves, and their foraging activities depend inside the forest. Thus, to maintain sustainable conservation of the endemic C.g.gallarum in the area, their habitats should be properly protected, and appropriate monitoring strategies should be designed.