Cymbopogon nardus Rendel (Family Gramineae) is widely distributed in tropical Asia. Its dried roots have been used in traditional Thai medicine for antipyretics, anti-inflammation, anti-analgesics and anticancer. The objectives of this study were to display the pharmacognostic specification, analyze the chemical constituents of essential oil and investigate the mutagenic and antimutagenic properties of C. nardus dried roots. Total ash, acid-insoluble ash, loss on drying and water content should be not more than 5.82, 3.97 , 8.29 and 8.06 % of dry weight respectively; while volatile oils, ethanol and water soluble extractive values should not be less than 2.96, 4.89 and 8.12 % of dry weight respectively. The major constituents of volatile oil from dried roots of C. nardus analyzed by GC/MS were elemol (22.87%) and alpha-eudesmol (16.09%). The mutagenic testing demonstrated that the ethanol and fractionated water extract of C. nardus roots were no mutagenic to S. typhimurium strains TA98 and TA100. In addition, both ethanol and water extracts of C. nardus roots at 15 mg/plate demonstrated strong antimutagenic effect toward S. typhimurium strains TA98 and TA100.