The volcanic rocks of the early Miocene Kori Formation and the latest Miocene Omosu Formation are distributed in the Yui area, western part of Oki-Dogo Island. The Kori Formation consists of conglomerate, sandstone and siltstone intercalated with basalt, basaltic andesite and andesite lavas and sheets. Most of these volcanic rocks are subalkaline（arc tholeiitic）. The volcanic rocks of the Omosu Formation is composed of shoshonite, rhyolite and trachyte lava and sheet with pyroclastic rocks. In the contrast with the volcanic rocks of the Kori Formation, the volcanic rocks of the Omosu Formation are characteristic of alkaline suite. This paper presents geology, petrography and K-Ar whole rock ages of the volcanic rocks of these formations. The volcanic rocks of the Kori Formation yielded K-Ar whole rock ages of 20.1±1.0Ma to 18.2±1.0Ma. The volcanic rocks of the Omosu Formation yielded whole rock K-Ar ages of 5.53±0.29Ma and 5.40±0.28Ma. These ages are consistent with the ages previously reported. The characters of volcanic activity changed from subalkaline and arc type in the early Miocene to alkaline and within plate（plume）type in the latest Miocene with hiatus of igneous activity during middle Miocene.