Abstract The composition of the essential oils from the aerial parts of three Labiatae species of Iran: Teucrium persicum Boiss., Thymus caucasicus Willd. ex Ronniger subsp. grossheimii (Ronniger) Jalas. and Marrubium crassidens Boiss. obtained by hydrodistillation were analyzed by GC and GC/MS. epi-α-Cadinol (23.2%) and α-pinene (17.3%) were the main components among the thirty-one constituents characterized in the oil of Teucrium persicum representing 95.9% of the total components detected. Seventeen compounds were identified in the oil of Thymus caucasicus representing 99.6% of the total oil with thymol (34.2%), methyl chavicol (25.1%) and γ-terpinene (12.7%) as the major constituents. The oil of Marrubium crassidens was characterized by higher amount of β-caryophyllene (20.3%), germacrene D (12.9%), caryophyllene oxide (11.1%) and cubenol (11.0%) among the fourteen compounds comprising 91.7% of the total oil detected. The oils (Teucrium persicum and Marrubium crassidens) consisted mainly of sesquiterpenes, while in Thymus caucasicus oil monoterpenes predominated over sesquiterpenes. Key Word Index: Teucrium persicum Thymus caucasicus Marrubium crassidens Lamiaceaeessential oil compositionepi-α-cadinolthymolmethyl chavicolγ-terpineneβ-caryophyllenegermacrene Dcaryophyllene oxidecubenol