Population dynamics of an introduced tree species has substantial importance for predicting forest ecosystem in oceanic island. Ryukyu pine (Pinus luchuensis) was introduced in 1900s from Ryukyu Islands to two oceanic islands, Daito and Ogasawara (Bonin) Islands. The pine had been used as firewood and/or windbreaks in these islands. However, the pine forests turned to be quite different secondary forests due to different anthropogenic and natural histories. The present study, thus, focuses on population dynamics of the introduced pine forests during 120 years based on field survey and tree ring analyses. Ryukyu pines on Chichijima Island are rather small and survived no longer than 60 years. Most of old pine before 1950s in Chichijima were died caused by attack from pine wood nematode in 1970s. After pine mass dieback, the population in Chichijima gradually recovered occupying vacant niche after 1980s. Recently pines have been facing decreases by control and extermination for conserving endemic species and ecosystem. In contrast, Ryukyu pines on Minamidaitojima Island have large basal-area and have grown since 1920s. Absence of younger trees after 1970s is another notable feature in this population. In Minamidaitojima where there haven’t been invasion of the nematode, pine forest has maintained a vital role as windbreak to protect from frequent damage by typhoon for the island.