Twenty-five essential oils and their major chemical components were screened for their possible antioxidant activities by assaying their DPPH free-radical scavenging activity (DFRS), total phenolic contents (TPC), trolox equivalent antioxidant capacity (TEAC), and ferric thiocyanate (FTC). Based on the TPC and TEAC assays, the essential oil ajowan is among the best essential oils studied. Furthermore, the DFRS and FTC assays reveal that the essential oils cinnamon bark extra and oregano are also among the best oils studied. More specifically, at a concentration of 1 mg ml(-1), the essential oils cinnamon bark extra and benzoin showed 93.75 ± 0.01% and 90.64 ± 0.01% DFRS, while the essential oils ajowan and oregano showed TEAC values of 4374.72 ± 0.01 and 4023.49 ± 0.01 μM of trolox per mg, respectively. In addition, the essential oils oregano and ajowan showed 29.17 ± 0.02% and 25.26 ± 0.03% FTC based on the assay results. At a concentration of 10 mg ml(-1), the essential oils ajowan and oregano showed 1845.20 ± 0.04 and 1665.36 ± 0.04 μg of TPC relative to GAE, respectively. Two major chemical components of the essential oils cinnamon bark extra, ajowan, and oregano were trans-cinnamaldehyde (90.61%), eugenol (2.58%), carvacrol (61.20%), p-cymene (37.44%), thymol (77.09%), and p-cymene (10.01%). It is clear that phenolic compounds in the aforementioned essential oils yield a positive correlation with the DFRS, TPC, TEAC, and FTC assays.