Grivet monkeys are mostly herbivorous and feed on a wide range of food items. A study on population size, activity time budget and feeding ecology of grivet monkey (Chlorocebus aethiops) was conducted in Sekele Mariam forest from March to August 2023 including the wet and dry seasons. Population census was conducted using direct count in blocks in four habitats. The activity time budget and feeding ecology of the study animal was conducted using instantaneous scan sampling. The data were analyzed using SPSS software and descriptive statistics and compared by using statistical analysis. The activity data was conducted on selected focal group of C.aethiops at the Sekele Mariam. Overall, the study found a mean population size, mean ± SD (247±41) with a minimum and maximum of 218 and 276 individuals of grivet monkey during the wet and dry seasons, respectively. Of which the highest number was accounted by juveniles 80(36.7%) followed by adult females 68(31.2%) during the wet, and juveniles 98 (35.5%) and adult females 89(32.3%) during the dry seasons. From the total observation, the study group spent 34.9% of its time for feeding, 22.2% for resting, 21.4% for moving, 20.7% for socialization and 0.9% for sexual activity. Feeding was the highest activity time budget of the study group during both the dry (34.5%) and wet (35.4%) seasons, followed by resting with the highest observation during the wet (22.2%) and dry (22%) seasons. They spent more time for socialization during dry season (21.8 %) than wet season (19.6 %) (P > 0.05). The food items consumed by the study group comprised the highest proportion of fruits 39.7% and followed by young leaves 36%, stem (20.6%) and invertebrates (3.7%). Fruit contributed the highest (71.52%) proportion during the dry season compared to the wet season (7.71%). While other food items contributed lower during the dry season compared to the wet season (P < 0.05). Rosa abyssinica (23.13%) was the most frequently food species consumed by grivet monkeys followed by Carissa spinarum (17.9%), Rhusg jutinosa (16.5%) and Achanthus senni (3.5%) was consumed during the dry season. On the other hand, Rosa abyssinica comprised the highest (20.9%) followed by Carissa spinarum (18.3%) and Achanthus senni (3.1%) was consumed during the wet season. Grivet monkeys slightly change feeding different food items at different season. Keywords: Behavioral activity, Feeding ecology, Forest, Grivet monkey, Population size