The essential oil (EO) obtained from Cymbopogon nardus is used frequently as ingredients of antiAIDS medication in Africa by indigenous people who rely mainly on traditional medicinal plant therapies. However, its mechanism still remains obscure. The present study investigated the biological effects of the EO derived from C. nardus collected in Benin on the activities of human immunodeficiency virus reverse transcriptase (HIV-1 RT) and Escherichia coli DNA polymerase I (DNA-POL I). Biochemical experiments showed that C. nardus EO inhibited the activity of HIV-1 RT, whereas it had no effect on DNA-POL I. To investigate the specificity of these biological effects of C. nardus, six other representatives EOs were assayed in the same way in order to determine their effects on HIV-1 RT and DNA-POL I. None of the other six EOs had significant effect on the activities of these two enzymes. Gas chromatography-mass spectrometry analysis of the EO obtained from the aerial parts of C. nardus by steam distillation identified among other constituents: (S)-βcitronellol, (R)-linalool, (R)-limonene, geraniol and (R)-citronellal. In particular, (S)-β-citronellol inhibited HIV-1 RT in vitro biochemical assay where the same concentration as C. nardus EO was used in the experiment.