This study is designed to examine the chemical composition and in vitro antioxidant activity of the essential oil and various extracts of Origanum rotundifolium Boiss. from Turkey. The total of the 40 identified components accounted for 97.23% (GC) and 95.10% (FID) of the total oil. Major components of the oil were borneol, terpinen-4-ol, and spathulenol. To determine the presence of well known and widespread phenolic compounds, water-soluble extract of O. rotundifolium was analyzed by HPLC. In the methanol-soluble extract, caffeic acid, p-coumaric acid, (+)-catechin, and ferulic acid were found in several quantities. Antioxidant activity of the essential oil and extracts of O. rotundifolium has been determined by four different test systems namely DPPH, beta-carotene/linoleic acid, chelating effect and reducing power. A strong correlation was observed between the radical scavenging capacity and polarity of the extracts. The methanol-soluble extract which contains the most polar phytochemicals showed the strongest antioxidant effect in all test systems. As expected, amount of the total phenolics and flavonoids was very high in methanol-soluble extract.