Male mating competition is high when the dominant male is unable to monopolize access to receptive females. Male reproductive success is not likely to be correlated with the dominance rank, because orangutans have a solitary life style with highly overlapping home ranges. We investigated the mating behavior and reproductive success of male Bornean orangutans. We study free-ranging orangutans at Sepilok Orangutan Rehablitation Centre, Malaysia. Orangutans visit feeding platforms but their behavior has never been disturbed by human staff. In 2010 and 2011, we followed 4 adult males and 4 adult females. We also collected feces from 22 individuals, including 3 pairs of mother and infant, and analyzed paternity by using 14 microsatellite loci. We observed agonistic encounters among adult males and decided their dominance rank. Agonistic interactions occurred around receptive females, but the females mated with not only the dominant but also the subordinates, including a few cases of forced copulation. Our result suggests promiscuous mating pattern and even the most dominant male is unable to guard receptive females. It may accelerate male mating competition and lead to occasional forced copulations. We will also discuss male reproductive success.