Although many Thai medicinal plants have been used in the treatment of infectious diseases, relatively little is known about the effect of these plants on oral diseases. Development of using medicinal plants with antibacterial activities in adjunct to conventional periodontal treatment is of interest. Objective: The aim of this study was to screen antibacterial activity of Thai medicinal plants against Porphyromonas gingivalis. Methods: 1) Anacardium occidentale leaves and bark, Terminalia bellerica seed, Syzygium cumini leaves and bark, Punica granutum pericarp, and Rhinacanthus nasutus leaves were extracted with ethy alcohol. 2) Screening the antibacterial activity against Porphyromonas gingivalis by plate diffusion method. 3) Conducting the minimal inhibitory concentration (MIC) of the extracts by two-fold broth microdilution method and 4) Evaluating the minimal bactericidal concentration (MBC). Metronidazole was used as a control. Results: The results showed that all of medicinal plant extracts were active against Porphyromonas gingivalis. The mean MIC of the studied substances were 0.00282, 1.068, 2.652, 4.687, 5.0, 5.95, 7.5, and 25.0 mg/ml for metronidazole, Anacardium occidentale bark, Anacardium occidentale leaves, Punica granutum pericarp, Terminalia bellerica seed, Syzygium cumini bark, Syzygium cumini leaves, and Rhinacanthus nasutus leave respectively. The mean MBC of the studied substances were 0.00312, 1.07, 1.754, 3.749, 5.625, 6.875, 20.0, and 25.0 mg/ml for metronidazole, Terminalia bellerica seed, Anacardium occidentale bark, Punica granutum pericarp, Anacardium occidentale leaves, Syzygium cumini leaves, Syzygium cumini bark, and Rhinacanthus nasutus leaves respectively. Conclusion: This in vitro study shows that all of the studied extracts were able to inhibit and kill Porphyromonas gingivalis but incomparative to metronidazole. Among the studied plants, Anacardium occidentale bark and leaves shows the best effect to inhibit Porphyromonas gingivalis and Terminalia bellerica seed shows the best effect to kill Porphyromonas gingivalis. This data further implicates that these plant extracts may be potential to use in periodontal therapy.