A study on the behavioural activities, group size and diet composition of gelada baboon (Theropithecus gelada) was conducted from August to November 2017 at Guassa Community Conservation Area (GCCA). Data were collected through direct observation of the study populations (both one male units OMUs, and all male units AMUs) using scan sampling method. The data were analyzed using descriptive statistics. The combined data for OMUs and AMUs showed that feeding was the rnDominant activity (35.70%), followed by moving (26.83 %), and resting (15.93%), while drinking (1.95%) and mating (0.48%) were the least frequently observed activities. OMUs had a mean group size of 11.25 dominated by adult females (mean=4.5) while the mean group size for AMUs was 6.6. Adult females of OMUs spent more time feeding (21.28%) than the leader male (17.35%) while the leader male spent the highest time resting (32.14%) compared to other age and sex categories. OMUs showed more grooming (61.3%), mating (71.88%) and resting (61%) during the morning while feeding (64.5%) and drinking (48.72%) were more frequent in the afternoon. Similar to OMUs, feeding (43.33%) and moving (32.19%) were more frequent in the dry season while resting (22.58%) and grooming (15.13%) were more frequent in the wet season. AMUs exhibited more resting (65.85%) and grooming (60.65%) in the morning while feeding (61.32%) and drinking (52.83%) were more frequent in the afternoon. A total of 12 species of plants were consumed by the geladas and grass Blades constituted the highest proportion (79%).