A new abietane diterpenoid, 12-O-methyl carnosol (2), was isolated from the leaves of sage (Salvia officinalis L.), together with 11 abietane diterpenoids, 3 apianane terpenoids, 1 anthraquinone, and 8 flavonoids. Antioxidant activity of these compounds along with 4 flavonoids isolated from thyme (Thymus vulgaris L.) was evaluated by the oil stability index method using a model substrate oil including methyl linoleate in silicone oil at 90 degrees C. Carnosol, rosmanol, epirosmanol, isorosmanol, galdosol, and carnosic acid exhibited remarkably strong activity, which was comparable to that of alpha-tocopherol. The activity of miltirone, atuntzensin A, luteolin, 7-O-methyl luteolin, and eupafolin was comparable to that of butylated hydroxytoluene. The activity of these compounds was mainly due to the presence of ortho-dihydroxy groups. The 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity of these compounds showed the similar result.