Abstract: In this paper, the authors investigated the petrology and zircon U-Pb geochronology of the intrusive rocks collected from the Inexpressible Island, Northern Victoria Land of the Antarctic Ross Orogen. The collected intrusive rocks are massive and nonfoliated quartz monzonite and quartz monzodiorite. LA-ICP-MS analyses yielded zircon U-Pb ages of 482.4Ma±4.2Ma and 484.3Ma±2.5Ma for the quartz monzonite and 484.0Ma±3.0Ma for the quartz monzodiorite. Both types of the rocks were emplaced and crystallized during the early stage of the early Ordovician. The magma of the quartz monzonite and the quartz monzodiorite should have been emplaced in an extensional setting. In conjunction with the previously reported data in the adjacent areas, it could be inferred that in the Northern Victoria Land the Ross Orogen developed mainly in the Cambrian and transformed to the late stage of orogeny or post-tectonic regime before the early stage of the early Ordovician.