In this study, aerial parts of Trifolium longidentatum Nâbelek (Fabaceae) were quantitatively analyzed for their content of four isoflavones, daidzein (D), genistein (G), fonnononetin (F) and bioclianin A (B), by the use of deve-loped RP-HPLC-UV method. The chromatographic separation was perfonned using a gradient system with a mobile pliase of metlianol, acetate buffer (pH=4.75) and acetonitrile with on the reversed pliase column, Agilent Eclipse XDB-18 (15 cm x 4.6 mm i.d.). The contents of D, G, F and B in the aerial parts of T. longidentatum were determined as 0.0095, 0.0056, 0.0021 and 0.0073 %, respectively, Antioxidant potential of the metlianol extract prepared from the aerial parts of the plant was also investigated with different in vitro metliods, namely DPPH radical scavenging activity, lipid peroxidation inliibition and trolox equivalent antioxidant capacity assays. As a consequence, it was observed tliat the metlianol extract of T. longidentatum had in vitro antioxidant activities in tliese metliods. The phenolic content of the extract was also measured as 53.98 ± 0.95 mg gallic acid/g extract by the Folin-Ciocalteu's reagent