The antimicrobial activity of water and ethanol extracts from 30 species of algae was measured using the agar diffusion method and minimum inhibitory concentration (MIC) test. In agar diffusion method, the 95% ethanol extracts from 12 of the algae showed growth inhibition against the tested microorganisms. In particular, Ishige okamurai, Ecklonia stolonifera, Sargassum siliquastrum, Sargassum thunbergii, Colpomenia bullosa, and Ecklonia cava had strong antibacterial activities against Gram-positive bacteria at 4 mg/mL. In the results of the MIC test, S. siliquastrum showed the most antimicrobial activity, where its MIC values ranged from 0.005 to 0.0075% against Listeria monocytogenes, Clostridium perfringens, and Basillus subtilis. In the thermal stability test, for the ethanol extracts of I. okamurai, E. cava, S. siliquastrum, S. thunbergii, and C. bullosa, the extracts proved to maintain high antimicrobial activities when they were treated at $121^{circ}C$ for 15 min. In the pH stability test, the antimicrobial activity of the S. siliquastrum ethanol extract was stable from pH 2 to 10, whereas the activity of the other species ethanol extracts were weakened under pH 10 against several microbes.