The study focused on the enzyme-assisted extraction and the purification of antioxidant phlorotannin content from brown algae Sargassum dupplicatum commonly grown in Vietnam. Antioxidant activities were evaluated, consisting of total antioxidant activity and reducing power activity for the enzyme-assisted extraction. Antioxidant purified phlorotannin was analyzed total antioxidant activity, reducing power activity, DPPH free radical scavenging activity, and lipoxygenase inhibition activity. The treating condition of brown algae by enzyme consisting of enzyme kinds (termamyl enzyme, cellulose enzyme, viscozyme enzyme), the enzyme concentration (2.5, 5.0, 7.5, 10.0, 12.5, and 15.0% (v/v)), and the treating time of algae (1, 2, 3, 4, and 5 hours). Brown algae were then soaked in 96% ethanol with the ethanol-to-brown algae ratio of 30/1 (v/w) for 24 hours at the room temperature for antioxidant phlorotannin extraction that purified by using the liquid-to-liquid method and the gel chromatography of Sephadex LH 20. Antioxidant phlorotannin purification was determined to base on DEPT and 1H NMR spectrum. The results showed that 7.5% cellulase enzyme destroyed brown algae cell for 3 hours leading the best efficiency of antioxidant phlorotannin extraction from brown algae for 24 hours by 96% ethanol with the ethanol-to-brown algae ratio of 30/1 (v/w) at the room temperature. Phlorotannin content, total antioxidant activity, and reducing power activity corresponded to 4.45±0.11 mg phloroglucinol equivalent/g DW, 11.17±0.28 mg ascorbic acid equivalent/g DW, and 11.09±0.24 mg FeSO4 equivalent/g DW, respectively. Two fractions of antioxidant phlorotannin (17 to 28 and 36 to 45) were collected after purification, possessed 4.1 to 177.3 and 8.3 to 112.2 mg phloroglucinol equivalent per 1087.56 mg glue, respectively. Total antioxidant activity, reducing power, and DPPH free radical scavenging corresponded to 12.48 to 585.52 and 28.08 to 371.28 mg ascorbic acid equivalent/1087.56 mg glue, 15.6 to 765.44 and 33.28 to 486.72 mg FeSO4 equivalent/1087.56 mg glue, and 72.23 to 100% and 76.02 – 100%, respectively. Lipoxygenase enzyme inhibition activity got 62.83 to 87.4 µM and 66.19 to 87.09 µM linoleic acid equivalent/ 100µl extract, respectively. DEPT and 1H NMR spectrum showed phloroglucinol exist in two over fractions.