Balanus rostratus is known to be a barnacle with simultaneous hermaphrodite. In this study, we tried to examine the sexual allocation pattern in B. rostratus with focusing on their body size and the number of neighborhoods (NN). We measured dry weights of reproduction organs and body size of B. rostratus in breeding season. Best model was then selected to determine factors affecting these reproductive organs. Full model included the following explanatory variables; (1) NN, (2) body size, (3) dissected month, and (4) interactions between body size and NN and (5) between body size and dissected month. In the best model, while ovary was explained by all variables, testis-seminal vesicle (TS) were explained by (1)-(4). The effects of NN on ovary and TS depended on body size. In larger individuals, the reproductive organ was larger with increase of NN. Neighborhoods are regarded as both potential partners for mating and competitors for space of substratum. Small individuals with more neighborhoods may invest more energy in growth to have space. We also observed individual moved chip of tergum and suctum alongside of a shell margin. B. rostratus may gather information about body size and NN from sounds and/or vibration occurred by the behavior.