Phenotypic dimorphism in exaggerated ornaments such as horns or mandibles in beetles has been paid much attention in the context of intraspecific competition and sexual selection. In stag beetles, for example, such dimorphism can be frequently seen in the same populations. In an Oedemeridae beetle Oedemera sexualis, while males possess thick hind femurs in most of the populations, those in some populations have thin femurs. From phylogenetic point of view, it is intriguing issue to elucidate when such dimorphism has been established and how the feature relates to the mating behavior. Here we aimed to examine how dimorphism in male femurs and in copulatory behavior has evolved based on phylogenetic comparative methods and geometric morphometrics. We have collected specimens from various localities in western Japan. We then conducted measurements of linear distance in several body parts and compared allometric coefficients of hind legs among local populations. Phylogenetic relationships among localities were inferred from mitochondrial and nuclear markers. The dimensional features of elaborated femurs were assessed using elliptical Fourier method. In the phylogenetic analysis, it has been revealed that a whole population of O. sexualis consists of briefly three major clades. All individuals possessing thin femurs clustered together in the same clade and the thin feature had directly derived from the lineages of thick femurs. We will show the transformation series of the configuration in hind femurs in accordance with tree topologies and will discuss the strength of phylogenetic signal in the deformation of femurs.