The chemical composition of essential oil of Cyperus rotundus L. grown in Iran was studied by hydrodistillation and GC and GC-MS. Analysis of the chemical composition of the oil showed the predominance of sesquiterpene compounds, with the presence of monoterpenes. Twenty-two compounds were identified from oil of C. rotundus, of which cyperene (37.9 %) and cyperotundone (11.2 %) were the major components.