In the studies conducted in 2010–2011, the chemical composition of the essential oils of three sweet basil (Ocimum basilicum L.) cultivars were investigated by GC-MS method. Forty eight compounds (95.08% of the total oil) were identified in the essential oil of O basilicum L. cv ‘Thai Siam’. The main components found in the oil were linalool (24.60 and 36.60% in 2010 and 2011, respectively), (E)-methyl cinnamate (18.73 and 21.90%) and methyl chavicol (5.57 and 7.50%). In the essential oil of O basilicum L. cv ‘Bolloso Napoletano’, forty constituents were identified, representing 97% of total oil. Linalool (41.09 and 47.75%), methyl chavicol (14.34 and 20.21%) and 1,8-cineole (7.21 and 10.23%) were the major components. Similarly, linalool (37.51 and 48.65%), methyl chavicol (13.41 and 18.55%) and 1,8-cineole (7.72 and 12.59%) were found to be the main constituents of the oil of O basilicum L. cv ‘A Foglie di Lattuga’. A total of 35 compounds were identified in this oil, representing 95.24% of the total oil content.