This study was conducted to investigate the flora of Mt. Seongdeok (Ganghwa-gun) in Korea. From the results of nine field surveys from March 2020 to July 2022, a total of 445 total taxa, representing 397 species, nine subspecies, 35 varieties, two forms and two hybrids were identified, which were placed in 273 genera and 93 families. The plant formation of Mt. Seongdeok is a deciduous broad-leaved and conifer mixed forest, which is common in the middle part of the Korean Peninsula. Most of the mountain is covered by a young secondary forest, which is mainly composed of Quercus ssp. and Pinus ssp. Among them, five taxa were endemic to Korea, one taxon were endangered plant and two taxa were red list plants. The floristic target plants amounted to 27 taxa, specifically one taxon of grade V, four taxa of grade III, seven taxa of grade II, and 15 taxa of grade I. And 43 taxa were northern lineage plants. A total of 46 species of alien plants were identified, with a Naturalized Index of 10.3%, an Urbanization Index of 11.7%, and six plants that disturbed the ecosystem. Our results provide basic data on vascular plants flora, and plant diversity and distributional changes.