ABSTRAK Penelitian tentang “Perilaku Bersarang Orangutan Sumatera (Pongo abelii)” telah dilakukan di Stasiun Reintroduksi Orangutan Jantho, Kabupaten Aceh Besar mulai April hingga Desember 2014. Penelitian ini bertujuan untuk mengumpulkan data perilaku bersarang orangutan sumatera (Pongo abelii) reintroduksi. Penelitian dilakukan dengan menggunakan metode Focal Animal Sampling. Parameter yang diukur adalah posisi sarang, tinggi pohon sarang dan tinggi sarang, lama waktu pembuatan sarang dan status sarang serta jenis pohon sarang. Data yang diperoleh dianalisis secara deskriptif yang ditampilkan dalam bentuk tabel dan gambar. Hasil penelitian menunjukkan bahwa persentase perilaku bersarang sebanyak 0,73%, makan 45,08%, bergerak 13,76%, dan istirahat 40,43%. Tahapan-tahapan pembuatan sarang orangutan sumatera yaitu pemilihan pohon sarang, pembuatan pondasi sarang, pembuatan matras, penguncian dan penambahan asesoris. Posisi sarang yang ditemukan selama penelitian adalah posisi 2 (38,67%), posisi 3 (36%), posisi 1 (24%) dan posisi 4 (1,33%). Tinggi pohon sarang yang sering digunakan adalah kategori 10+ hingga 25+ dan tinggi sarang adalah kategori 5+ hingga 20+. Waktu untuk menyelesaikan pembuatan satu sarang baru berkisar antar 1 hingga 6 menit dan perbaikan sarang lama berkisar antara 1 hingga 4 menit. Sarang orangutan sumatera paling banyak dibangun pada pohon Eugenia sp. (11 sarang) dan Mallotus sp. (8 sarang) dari total 75 sarang yang didapatkan. Kata kunci: orangutan sumatera (Pongo abelii), perilaku bersarang, reintroduksi, Jantho. ABSTRACT The research about “Nesting Behavior of Sumatran Orangutan (Pongo abelii)” had been done in Orangutan Reintroduction Station of Jantho, Aceh Besar District from April to December 2014. This research aimed to collect data nesting behavior of sumatran orangutan (Pongo abelii) reintroduction. The research was conducted by using the Focal Animal Sampling method. Measured parameter were the position of the nest, the nest tree height and nest height, the length of time for nesting and nest status and species of nest trees. Data were analyzed descriptively shown in the form of tables and images. The results showed that the percentage of nesting behavior as much as 0.73%, feeding 45.08%, moving 13.76%, resting 40.43%. The stages of construction of the sumatran orangutan nests were the choice of nest trees, nest foundation construction, construction of mattresses, locking and additional accessories. The position of nests were found during the study was the position 2 (38.67%), position 3 (36%), position 1 (24%) and position 4 (1.33%). Height of nest tree that was often used were the categories of 10+ to 25+ and height of nest were 5+ to 20+ categories. Length of time to complete construction of a new nest ranged between 1 to 6 minutes and repair old nest ranged from 1 to 4 minutes. Sumatran orangutan nests were built on tree at most Eugenia sp. (11 nests) and Mallotus sp. (8 nests) from total of 75 nests those were found. Keywords: Sumatran orangutan (Pongo abelii), nesting behavior, reintroduction, Jantho.