Forty-five kinds of commonly used essential oils were employed to investigate the DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging ability and total phenolic content of major chemical compositions. The free-radical scavenging ability and total phenolic content of cinnamon leaf and clove bud essential oils are the best among these essential oils. One-half milliliter of cinnamon leaf and clove bud essential oils (10 mg/ml EtOH) are shown to be 96.74% and 96.12% of the DPPH (2.5 ml, 1.52x10(-4) M) free-radical scavenging ability, respectively. Their EC50 (effective concentrations) are 53 and 36 (microg/ml). One milligram per milliliter of cinnamon leaf, clove bud, and thyme red essential oils were shown to be 420, 480, and 270 (mg/g of GAE) of total phenolic content, respectively. Eugenol in cinnamon leaf and clove bud essential oils (82.87% and 82.32%, respectively) were analyzed by GC-MS. It is clear that the amounts of the phenol compounds in essential oils and the DPPH free-radical scavenging ability are in direct proportion.