Objectives: The purpose of the study was to investigate the antimicrobial activities of the essential oil from Cymbopogon citratus (lemongrass) against some periodontal pathogens. Methods: Such pathogens included Actinomyces naeslundii (WVU45), Porphyromonas gingivalis (W50), and the clinical isolates from 3 gingivitis and 3 periodontitis patients. The activities were performed by using the broth dilution and antibiotic sensitivity tests. Results: The results revealed that the minimum inhibitory concentration (MIC) of the essential oil against Actinomyces naeslundii and Porphyromonas gingivalis were 0.44 and 0.22 mg/ml, respectively. Furthermore, the results showed that some bacterial strains from clinical isolates could resist the activities of both concentrations. However, the black-pigmenting bacteria could not be detected in any group. These resistant bacteria could be differentiated into 10 different groups depending upon their antibiotic sensitivity patterns to the following four antibiotics, tetracycline hydrochloride, ceftazidime, ampicillin, and erythromycin. Five of the 10 groups were susceptible to the tested oil at the original concentration, wheras only one group was sensitive to 10% tetracycline hydrochloride. Conclusions: It can be concluded that Cymbopogon citratus has activities against both reference strains and the major groups of the clinical isolates, especially the tetracycline hydrochloride-resistant strains. The present study suggested the benefit of the using essential oil to treat any tetracycline hydrochloride-resistant bacteria in combination with other antibiotics in future studies.