Abstrak. Telah dilakukan penelitian uji aktivitas antioksidan daun gulma siam (Chromoleana odorata L.) dengan metode 1,1-difenil-2-pikrilhidrazil (DPPH). Tujuan penelitian ini untuk mengetahui kandungan senyawa metabolit sekunder, golongan flavonoid dan aktivitas antioksidan yang terdapat dalam daun gulma siam. Sebanyak 315 g daun gulma siam kering yang sudah dihaluskan, dimaserasi dengan etanol 95% diperoleh 100 g ekstrak kental etanol. Sebanyak 70 g ekstrak kental etanol difraksinasi dengan pelarut metanol dan n-heksana menghasilkan fraksi kental metanol 30,72 g dan n-heksana 14,62 g. Hasil skrining fitokimia ekstrak etanol menunjukkan positif mengandung senyawa alkaloid, flavonoid, saponin, tanin dan steroid. Fraksi metanol positif mengandung senyawa alkaloid, flavonoid, saponin, dan tanin. Fraksi metanol dilakukan isolasi dengan metode kromatografi kolom menggunakan eluen n-heksana : etil asetat : diklorometana (1:1:1) sebagai fase gerak pertama, kemudian ditingkatkan gradien kepolarannya untuk menghasilkan pemisahan secara sempurna. Hasil isolasi diperoleh 8 fraksi ( Abstract. The antioxidant activity of siam weed leaf (Chromoleana odorata L.) has been done with 1,1-diphenyl-2-picrilhidrazil (DPPH) method. This study is intended to know the the content of secondary metabolite compounds, the group of flavonoid and antioxidant activity that found in siam weed leaf. The 315 g dry siam weed leaf that was refined, macerated with ethanol 96% produced 100 g of ethanol viscous extract. The 70 of ethanol viscous extract was fractionated with methanol and n-hexane solvent produced 30,72 g of methanol viscous fraction and 14,62 g nhexane. The results of phytochemical screening of ethanol extract showed positive that contained compound of alkaloids, flavonoids, saponins, tannins and steroids. Methanol fraction showed positive that contained compound of alkaloids, flavonoids, saponins and tannins. The isolation in methanol fraction was made by coloumn chomatography method using eluent n-hexene : acetate ethyl : dichloromethane (1:1:1) as first mation phase. Then the polarity gradient was increased to produce complete separation. The result of isolation produced 8 fractions (A, B, C, D, E, F, G, H) and the analysis results with TLC showed fraction of A and B was positive contain flavonoids compound. Flaconoids compound that was contained in fraction A was estimated isoflvone group and fraction B was estimated group os isoflavone, flavone, flavonol and chalcone. Analysis FTIR showed that the fraction of A and B containing function group N-H, O-H (carboxylic acid), C=O (aldehyde and ester), C=C, C-H, C-X (flouride), C-O and C-N. The result of antioxidant activity test used DPPH method, obtained inhibitor concentration (IC50) for 15,5067 ppm of ethanol extract, 9,5671 of methanol fraction, 82,7808 ppm of fraction A and 16,2336 ppm of fraction B, while for comparison (ascorbic acid) is 0,8913 ppm. This, it can be concluded that strongert antioxidant activity was found in methanol fraction.