The Ryukyu Islands (Amami Islands, Okinawa Islands and Sakishima Islands) form an island arch situated at the southern end of the Japanese Archipelago. In this area, there are numerous fossiliferous localities estimated to date from the end of the Pleistocene, which have yielded abundant vertebrate fossil remains. Among the excavated fossils of terrestrial vertebrates, two extinct deer species, Cervus astylodon and Muntiacinae gen. et sp. indet., are representative of the Late Pleistocene fauna of the Okinawa Islands. However, their ecological characteristics have been largely unknown to date. In the present study, we reconstructed the paleoecology of the deer species excavated from the Hananda-Gama Cave, Okinawa Island. By mesowear analysis and stable isotope analysis of the fossil deer molars, it was estimated that both deer predominantly consumed C3 browse under the climatological/environmental condition similar to the current Ryukyu Islands. Additionally, using tooth wear model of extant sika deer population with a similar diet, the age of C. astylodon was estimated. The fossil deer were estimated to have lived up to 26 years, which was surprisingly long for their small body size. Life history of C. astylodon was considered to follow a K-selection strategy (or slow life history), which might have evolved under predator-free insular environment. This life history trait made them vulnerable to human exploitation, because animals with a K-selection strategy are represented by lower population recruitment. Human hunting might have a considerable impact on the fossil deer populations, and be responsible for their extinction at the end of Pleistocene.