In sex-role reversed species, females compete with the same-sexed individuals for resources more intensively than males do. Cichlid fish in genus Julidochromis exhibit intra-specific variation in mating system, with some species showing facultative polyandry and sex-role reversal. It remains unclear whether those species have sex difference in aggressiveness in the context of within-sex contests. In this study, we observed aggressive interactions among three same-sexed individuals of J. regani. Although larger individuals usually attack smaller ones in cichlids, aggression by a smaller individual towards a larger one is occasionally observed in this species. Therefore, we examined the effects of size difference, sex and the direction of aggression. We found a significant three-way interaction among those factors. Regarding sex difference, we found (1) the frequency of aggression by small individuals against large ones occurred more frequently among females than males; (2) the frequency of aggression by large individuals against small ones decreased as size difference increased in males, but was invariant among females. The sex difference in aggressiveness might be related to the facultative polyandry in which females’ coexistence is rare.