Sexual dimorphism, such as sexual size dimorphism (SSD) and sexually dimorphic exaggerated traits, often evolves via sexual selection. In many species, evolution of sexual dimorphism is thought to be driven by either of the two forms of sexual selection: intra- and inter-sexual selection. In some species, however, intra- and inter-sexual selection act simultaneously on the same sexually dimorphic trait. Therefore, it is important to consider the effects of both forms of sexual selection to fully understand the evolution of sexual dimorphism. Drosophila prolongata is a fruit fly that shows male-biased SSD and has enlarged forelegs only in males. In this study, the relationship between body size parameters and aggression/mating behavior was examined. Our results showed that aggressive behavior was influenced by body weight and foreleg size, whereas mating success was not influenced by any size parameters, suggesting that intra-sexual selection is the primary mechanism that maintains the sexual dimorphism in the current D. prolongata population.