Background: By studying population size, activity patterns, diet, and ranging ecology of Boutourolini’s blue monkey (Cercopithecus mitis boutourlinii) we can get sufficient information to conserve the subspecies in the area. Boutourlini’s blue monkey is endemic subspecies found in the western and northwestern parts of Ethiopia. The study was conducted in Apini and Dokuma forests, northwestern Ethiopia, from October 2018 to June 2019. The block count method was used to estimate its total population size and scan sampling method was used to collect data for activity patterns, and diet. The ranging ecology of the study subspecies was determined for each group based on the point to point movements of the group between consecutive GPS locations recorded. Results: On average a total of 71 and 111 individuals of Boutourolini’s blue monkey were counted in Apini and Dokuma forests, respectively. Boutourolini’s blue monkey spent 47.5% and 48.6% of time feeding, 20.2% and 18.6% moving, and 14.1% and 13.5% resting by the Apini and Dokuma groups, respectively. The Apini group frequently fed on young leaves (52.8%), fruits (30.2%), and mature leaves (6.6%), while the Dokuma group fed on young leaves (39.8%), fruit 942.3%), and mature leaves (8.3%). Ranging ecology of Boutourolini’s blue monkey was 44.4 ha and 78.3 ha for the Apini group, and 51 ha and 56.9 ha for the Dokuma group during the wet and dry seasons, respectively. Conclusions: The total population size of blue monkeys in the Apini and Dikuma forests counted were different. Activity budgets vary during the wet and dry seasons. During the entire study period blue monkeys consumed different food items from the two forests. The home range size of blue monkeys during the two seasons was different in both groups and they traveled long distance during the dry season. As the habitats of Boutourolini’s blue monkey in the present study area is degraded due to various anthropogenic activities, there is a need to design strategies to minimize conservation problems of the subspecies in the area.