Study on population size, diurnal activity budget, and human-grivet monkeys (Chlorocebus aethiops) conflict in Zegie Peninsula was carried out from August 2018 to March 2019, the study comprised both the wet and dry seasons. Line transect method was used to collect data on population size of grivet monkeys. Scan sampling method at 15 minutes interval was used to collect data on diurnal activity patterns and feeding ecology of grivet monkeys. Questionnaire and focus group discussions were used to study human-grivet monkey conflict and its conservation status. The estimated population of grivets in the study area was 4808. Population structure of grivets indicated that there were 634 adult males, 1236 adult females, 1733 juveniles, and 1205 infants. Grivet population estimate between the wet and dry seasons did not show significant differences (ᵡ 2 = 0.938, df = 1, p > 0.05). But there was significant difference in the population estimate of grivets among the different habitats (ᵡ 2 = 238.295, df = 3, p < 0.05). Grivet monkeys spent 39% of time feeding, 30% moving, 20% resting, 5% grooming, 3% social play, 2% aggression, and 1% for other activities. The overall diet of grivet monkeys was dominated by fruit (61%), while young leaf accounted (22%), and young shoot (8%). A total of 22 plant species were consumed by grivet monkeys in the study area. There was human-grivet monkey conflict in the Peninsula and grivets were damaged crops, vegetables, and fruits. They were also predating poultry, steal egg, and human food. Consequently grivets were killed in response to the damage they cause to crops. Grivet’s habitat in the Peninsula is highly disturbed because the local people cut trees for sell to support their livelihood. Awareness creation of the residents about wildlife and their habitat is necessary, and people should understand the impact of deforestation, illegal hunting and the role of grivets in the ecosystem at large. Hence there is a need to protect the forest in order to ensure sustainable conservation of biodiversity in general and grivet monkey in particular.