Geographical isolation in island species has been linked to concurrent variation in floral morphology and pollinator fauna, but analyses across large geographical areas are scarce in the literature. Oceanic islands provide ideal conditions to detect this link because of their isolation from the continent by the sea since their formation. Here, we selected the Izu Islands from several oceanic islands in Japan as the research site, and compared the floral morphologies and pollinator fauna of Ligustrum ovalifolium on the Japanese mainland (Honshu) and the Izu Islands to evaluate the relationships between them. In this study, we aimed to confirm the morphological differentiation of L. ovalifolium flowers between the two regions and to investigate the relationships between phenotypic variation in floral traits and pollinator diversity. The morphological analyses revealed the differentiation of L. ovalifolium flowers between Honshu and the Izu Islands. On the Izu Islands, corolla tube and stamen lengths were shorter than in the mainland relatives; furthermore, this shortening became more pronounced on Hachijo Island, the most isolated from the mainland. A similar pattern has been identified in other species endemic to the Izu Islands. Therefore, in L. ovalifolium, we examined floral morphological variations as in the insular plants studied previously. We found that the pollinator fauna on Hachijo Island, the southernmost island, was significantly different from that on both the other Izu Islands and mainland Honshu. These findings suggest close relationships between phenotypic variation in floral traits and pollinator fauna in L. ovalifolium on the mainland Honshu and the Izu Islands. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174, 489–501.