Antimicrobial activities were assayed through the hot-water extracts from 41 species of medicinal plants against Helicobacter pylori which is known as the ulcerogenic phathogen. Opuntia ficus-indica, Houttuynia cordata, Sinomenium acutum, and Coptis japonica showed the MIC at the concentrations less than 100 ppm, Pulsatilla koreana, Forsythia koreana, Rheum undulatum, and Perilla frutescens less than 200 ppm, Belamcanda chinensis, Arctium lappa, Cassia tora, Citrus tachibana, Siegesbeckia orientalis, and Caesalpinia sappan less than 300 ppm by the 2-fold dilution method. In disc method only three of them were confirmed to have antimicrobial activities which were increased in the order Perilla frutescens, Coptis japonica, Caesalpinia sappan. Three extracts were partitioned with chloroform, ethyl acetate and butanol in sequence and examined for the activity to inhibit H. pylori. The major ativities were observed in ethyl acetate fraction of Caesalpinia sappan, butanol fraction of Perilla frutescens, butanol and chloroform fraction of Coptis japonica. The partitioned fractions were found to have increased antimicrobial activities in all extracts. The experiments in which the extracts were added into urea R broth containing the crude urease derived from H. pylori resulted in the increase of pH and optical density at 560 nm to 8.15 and 1.7 respectively. Urease activity of H. pylori was inhibited over 80% by Caesalpinia sappan, Perilla frutescens, and Coptis japonica, of which Caesalpinia sappan suppressed up to 95%.