Bacteria Porphyromonas gingivalis is the most common gram negative bacteria in people with periodontitis. Porphyromonas gingivalis growth of nearly 85% meets inflammatory gums and is rarely found in healthy gums. Beside those synthetic medicaments, as an alternative non-synthetic medicament that can be applied is natural biological compound-related medicament harboring antibacterial potency. One of the candidate is ethanol extract of Siam weed. This study aims to assess the inhibitory of the Porphyromonas gingivalis due to the addition siam weed. This research was conducted by experimental laboratory in vitro. Liquid dilution method was used using Tryptose Phosphate Broth media while the measurement of turbidity was performed by using Spectrophotometer UV-vis. Extracts ethanol of siam weed were tested on the bacteria Porphyromonas gingivalis into some concentrations: 1.55%; 0,78%; 0.39 and 0.19% based on the weight/volume (w/v). All tested concentrations can inhibit the inhibitory rate of Porphyromonas gingivalis bacteria. The most effective concentration against bacterial growth rate was 0.78% and 1.55% (p <0.05). The etanol extract of Siam weed effectively inhibit the growth of Porphyromonas gingivalis bacteria.