Basil is an important medicinal and aromatic plant with very wide range of uses. Water stress has a crippling effect on growth and productivity of basil plants; therefore a pot experiment was carried out to study the response of growth and chemical composition of four basil varieties (Thai basil, Camphor basil, Red Rubin, and Lemon basil) to exogenous application of silicon (0, 25, 50 ppm) under different irrigation regimes (Irrigation at 3, 6, and 9 days intervals) in a split split-plot design. The results showed that although basil varieties varied significantly in their performance under the different irrigation regimes, but prolonging irrigation intervals had a significant negative effect on plant height, number of branches per plant, leaves/stem ratio, leaf area, fresh and dry herb per plant, chlorophyll content, essential oil percentage and essential oil yield per plant in all evaluated varieties. Chemical parameters in terms of total carbohydrates, total phenoles and proline contents increased with prolonging irrigation intervals in all varieties. Foliar application of silicon improved all growth and chemical parameters in all basil varieties especially under severe irrigation conditions. The best results were obtained when plants treated with 50 ppm of silicon. Methyl chavicol, linalool, eugenol were the major constituents of basil essential oil.