Aerial parts of Ocimum basilicum var. purpurascens Benth, Ocimum basilicum var. dianatnejadii Salimi at flowering stage were collected from plants grown in Experimental Station of Pykan Shahr, near Tehran. Elevation 1215 m above sea level, latitude 35° 42′ North, 51° 8′ East, average humidity 36% and climatic category semi-arid.

Methyl chavicol (43.0%) and linalool (28.9%) were identified as the major compounds in the oil of O. basilicum var. purpurascens, while methyl chavicol (37.6%), linalool (33.4%) and alpha-cadinol (5.7%) were the major constituents in the oil of O. basilicum var. dianatnejadii.

Seeds of Ocimum basilicum cv. keskenylevelu provided from Hungary, were used in this study. Potted seedlings of Ocimum basilicum were subjected to study the effect of different irrigation rigimes on the essential oil content and composition at experimental farm of college of agriculture, Tarbiat Modarres, University, located in Tehran. (1215 m above sea level, latitude 35° 43′ north, altitude 51° 8′ east). The seeds were sown in spring of 2001 in pots. The irrigation regimes to induce of water stress were: 100%, 85%, 70% and 55% of field capacity. This percentage of field capacity kept constant in the soil by daily weighting of pots. The soil was sandy-loam with 22.6% of field capacity. The harvest of whole plants was performed at the beginning of the flowering stage.

The essential oil content of herb increased from 1.12 to 1.26% as plant water deficit increased (till 70% of field capacity). The number of component of the oil of Ocimum basilicum increased as water stress increase. Amount of the main constituents of the oil such as linalool, methyl chavicol, 1,8-cineole and trans alpha-bergamotene significantly affected by water stress.