Abstract Formosan macaque (Macaca cyclopis) is the endemic and threatening species of Taiwan, but information regarding their social behaviors is rare. Troop fission of macaque¡¦s social behaviors was the rarely and accidental behavior. Two fission of Formosan macaques troops, from B0 and C0, at Mt. Longevity of Taiwan were the objects in this study. Field study was concentrated from November 1999 when fission occurred till May 2000. The records included the troop¡¦s composition changes and interactions of troops after fission, and using these records to infer the reason of troop fission in Formosan macaques. The focal sampling method and scan sampling method were used to collect individuals¡¦ behaviors¡Bsocial behaviors and habitat utilization of four troops¡]main troop B, branch troop Ba, main troop C and branch troop Ca¡^ after fission. Troop B and Ba remained separately six months after fission. But two months after fission, a fusion occurred between troop Ca and troop C. In the phenomenon of fission, the individuals who left the original troop and emigrated to the branch troop were all the subordinate or periphery individuals in the original troops. When they emigrated to the branch troops, their ranks were higher than that of the original troops. Adult females initiated the troop fission, and the other individuals joined later. Compared with the adult male, the composition of troop adult females were much stable and were the nucleus of branch troops. Only the adult sex ratios of troop Ca was lower than that of the original troop. Females delivered infants in branch troops were pregnant before troop fission. These two cases indicated that: fission occurred in Formosan macaque was not due to increasing mating partners or chances, but because of some adult females¡¦ fitness lost in their original troop. After fission, the branch troops, Ba and Ca, were 1/4 ~ 1/5 troop size to the main troop. When main troop B met the branch troop Ba, the behaviors of main troop B indicated troop B was dominant to the branch troop Ba. The main troop B and branch troop Ba had a significant difference in home range using ¡]£q2 = 49.195, d.f.= 14, p< 0.05¡^. The branch troop Ba¡¦ home range was larger than that of the main troop B or of the original troop B0. The difference of night resting-sites between the main troop B and the branch troop Ba was not significant¡]£q2 = 11.6, d.f.= 13, p> 0.598¡^.