Sumatran orangutans, found in the Province of Nangroe Aceh Darussalam and North Sumatra Province, are fragmented into four populations. One of these populations exists in Angkola, a forested area located in the southern part of Lake Toba. This population is considered small and narrowly distributed. Our survey shows that individual orangutans appear in and around the Dolok Sibualbuali Nature Reserve. This indicates that orangutans can adapt to fragmented habitats, such as those found near Dolok Sibualbuali Nature Reserve, south of Tapanuli, North Sumatra.The preliminary study was conducted from April 2002 to January 2003. This study recorded the distribution and daily activities of orangutans in Dolok Sibualbuali Nature Reserve. Repeated crisscrossing surveys and focal animal observation were used to document orangutan distribution and daily activity.Human activities are the main factor for the loss of orangutan habitat. Illegal logging, forest conversion for timber concessions and plantations, road construction, and settlement are the main factors for forest loss. This forces orangutans out of their native habitat and makes them vulnerable to extinction.Based on direct sightings, orangutans are found at Aek Nabara and Sitolu. Moreover, the presence of orangutans is indicated by nests at the Sihulambu and Bulu Mario sites. The orangutan population is not evenly distributed but seems to be present at locations with food resources.Seven individuals were chosen as focal animals. They consisted of three adult females, three sub-adult males, and one juvenile. Total observation time was 757 hours and 40 minutes. Feeding, moving, resting, nesting, and social activities were the main activities that were recorded. Orangutans spent their daily activity budgets as follows: feeding (42.29%), moving (16.47%), resting (37.41%), nesting (1.89%), and social contact (1.93%). Orangutans spent much of their time (42.29%) feeding, particularly when food resources were abundant. Due to their large body size, food consumption was high.There are 115 types of orangutan food consisting of fruits (36.6%), flowers (3.89%), leaves (28.33%), bark (17.7%), and pith/stem (11.67%), and other food types (insects, honey, etc.) (1.67%). Fruit is the main source of orangutans' food.We concluded that the distribution and daily activity of orangutans at Dolok Sibualbuali Nature Reserve are greatly influenced by the availability of food, particularly fruit. When favored fruit is abundantly available, orangutans will constantly consume that fruit and visit fruit trees of the same species in other locations. Orangutans will also search for other food in community farms near the Nature Reserve when forest fruit availability is low, leading to conflict with local people.