Presence of Eastern Hoolock Gibbon (Hoolock leuconydes) was confirmed recently in and around Mehao Wildlife Sanctuary, Arunachal Pradesh, where the last remnant rainforest region of Eastern Himalaya still exists. A total of 13 groups of Eastern Hoolock Gibbon were located in this Lower Dibang valley in the outskirts and periferi of Mehao WLS where they coexist with humans at the non-protected agricultural landscape. The present study focuses on the recent behavioural modification of Hoolock Gibbon in the non-protected fragmented region coexisted with local population at the juxta region of agricultural landscape and groves. Satellite data indicates massive deforestation in last 30 years with a rate of 7 sq km per year leading to multiple fragmented forest patches with an average area of 200-800 sq meters. These patches are home for a family of Eastern Hoolock Gibbon consisting of with an average family size of 3.5. Agricultural expansion, developmental activities, tourism and migration of population to this part of Eastern Himalaya have significantly modify behavioural aspects of Eastern Hoolock Gibbon. Our observation shows a larger family size, descending to the ground and maize crop raiding as a result of food scarcity and habitat crunching due to the recent anthropogenic activities. The chances of human-animal conflict will expect to increase as a consequence of these behavioral changes. The conflict could be elevated due to the recent change in sociological demographics in the non-protected regions. A call for immediate action is needed for the survival of these species in the changing land use pattern over newly emerged eco-sociological dynamics.