Abstract: The Hainan gibbon (Nomascus hainanus) is one of the most endangered primates and the least studied gibbon species in the fieldwork. In the past decades only a little information was known about its population size, population structure, habitat and food kinds. Through four years field work, we observed their inter-group encounters for four times. We found that their encounter behaviors were much different from other gibbon species. Only vocal and chasing behavior happened between the adult females, male sub-adults and male juveniles of the Hainan gibbon encounters. The time of inter-group encounters was much shorter than other gibbon species, just 24-51 minutes. Not like the Hylobates lar encounters, no fighting was observed among the Hainan gibbon encounters; there was also no such behaviors as playing and grooming, even the extra-group-copulations. The adult females of two groups didn’t take par in the chasing and they only rested and watched 20-30m away. We speculated that the behavior of the adult females may be interpreted as protecting their territory and by the way of chasing the sub-adults can learn how to protect their territory in the future.