Because the costs and benefits of living in a group change according to ontogenetic stage, it is predicted that difference of body size will produce difference for social preferences even in same individuals. For increasing pay-off by group-living, it is predicted that small individuals in early developmental stages will cooperatively form groups to reduce predation risk, whereas large individuals will reject each other to avoid resource competition. Furthermore, when individuals can recognize kin, social preference might vary depending on kinship between each other. In this study, we examined the association preference between different-sized Rana ornativentris tadpoles, i.e., small individuals in early developmental stages and larger tadpoles at later developmental stages. Our binary-choice experiments showed that regardless of size, R. ornativentris tadpoles possess the ability to discriminate based on both phenotype and kin recognition. That is association preferences were determined by the size combination between a subject tadpole and each stimulus tadpole and by the interaction between kinship of a subject and each stimulus. Both small and large tadpoles spent a longer amount of time at the side of small tadpoles compared to large tadpoles. However, there was a significant difference in preference between small and large tadpoles: when opposite side were large non-siblings, small tadpoles tended to associate with both similar-sized small siblings and non-siblings. In contrast, large tadpoles showed no preference for small siblings.