Forty natural plants collected in Jeju, Jeonnam-Goheung, and Gyeongbuk-Ulleung were extracted using a pressurized liquid. Extraction yields of total soluble solids and total phenolics (TP), and integral antioxidative capacity (IAC) were measured, and antimicrobial activity was tested against Streptococcus parauberis, Streptococcus iniae, Edwardsiella tarda, and Vibrio ordalii. Jipsinnamul showed the highest content of TP (174.4 mg GAE/g), followed by Mulchamnamu (116.9), Seoeonamu (113.3), and Buknamu (108.2). Percent TP/TSS was high in Jipsinnamul (72.6%), Seoeonamu (47.3%), Mulchamnamu (46.4%), Jageumu (40.2%), and Baneulkkot (40.1%), respectively. Magamok, Nadosongipul, Buknamu, Mulchamnamu, and Seoeonamu showed 5.81, 3.96, 3.63, 3.63, and 3.34 mmol ascorbic acid equivalents/g of IAC of water-soluble substances, and Seoeonamu, Magamok, Seipijilpul, Mulchamnamu, Baneulkkot, and Seomgirincho showed 8.51, 6.57, 5.68, 3.85, 3.83, and 3.69 mmol Trolox equivalents/g of IAC of lipid-soluble substances, respectively. Only nine species such as Baneulkkot, Dokhwal, Jipsinnamul, Mulchamnamu, Nadosongipul, Seipijilpul, Seoeonamu, Seomgirincho, and Sumbadi of 40 selected plants showed the antimicrobial activity against four bacteria tested. Jipsinnamul showed the strong antimicrobial activity against S. iniae, while Dokhwal, Nadosongipul, and Sumbadi against S. parauberis and S. iniae, and Mulchamnamu, Seoeonamu, and Seipijilpul against V. ordalii.