In this work, the antioxidant and antimicrobial activities of Brassica oleracea var. capitata extracts obtained through ultrasound-assisted extraction are evaluated. The extracts obtained using the best extraction conditions were subjected to different hydrolysis conditions before their use in the biological tests. The crude and hydrolysed extracts were characterized using gas chromatography coupled with a mass detector. The use of ultrasound at 30 °C with 60% (by volume) solvent enabled obtaining a richer extract. All extracts had antioxidant activities against DPPH (13.0-80.0%), superoxide (35.2-63.2%) and peroxyl (89.3-99.5%) radicals, but the use of hydrolysed extracts considerably improved the antioxidant activities. Antimicrobial activities only of the hydrolysed extracts of Brassica oleracea var. capitata were detected. It was confirmed that antioxidant activity of vegetable extracts can be considerably increased when hydrolysis is applied as a pretreatment to their extraction.