Inter-group conflicts are common among many group-living animals and involve potentially complex motivations and interactions. Mammals living in multilevel societies offer a good opportunity to study inter-group conflicts. This study is the first to explore the function of sex-specific participation during inter-group conflicts within a multilevel society at the individual level. The Sichuan snub-nosed monkey (Rhinopithecus roxellana) is an endangered seasonal breeding species living in a multilevel society. From Sep 2007 to May 2008 we recorded 290 inter-group conflicts of a free-ranging provisioned band of R. roxellana in the Qinling Mountains of China to investigate the function of individual aggression during inter-group encounters. Our findings show that adult males were the main participants in inter-group conflicts, while females took part in them only occasionally. The male participation rate during the mating season, when adult females were estrous, was significantly higher than that during the non-mating season. Furthermore, males directed their aggression to other males, and directed more intense aggression towards bachelor males than towards other resident males. For both sexes, the participation rate as initiators was higher in the winter than that in the spring; and there was a significant positive correlation between group size and the participation rate as initiators. Our results suggest that inter-group aggression in Sichuan snub-nosed monkeys is linked to both mate defense and resource defense.