The aim of the paper was to evaluate the effect of hydrothermal treatment on total content and antioxidant activity of phenolic acids isolated from broccoli. The polyphenol compounds were extracted from fresh and frozen broccoli (before and after boiling) with 50% methanol. After removing the solvent under reduced pressure the phenolic acids were extracted with diethyl ether:ethyl acetate (1:1) mixture from water extract. The obtained extracts were chromatographed on a Sephadex LH 20 column. The total phenolic acids content and their ability of inhibiting linoleic acid autooxidation was determined. A similar analysis was conducted of the water residue after boiling. In the extract obtained from raw fresh broccoli the synapic, ferulic, caffeic and syringic acids were identified. All phenolic acids were identified in the extracts from boiled broccoli and in the water residue. The raw frozen broccoli include synapic, ferulic, caffeic and syringic acid. All phenolic acids were identified in water residue, whereas the boiled frozen broccoli included only ferulic and syringic acid. The phenolic acids content decreased (80%) after cooking raw unfrozen broccoli. The water residue from fresh broccoli had a higher the phenolic acids content than the water residue from frozen vegetable. Samples obtained from raw broccoli were characterized by a lower ability of inhibiting linoleic acid autooxidation than samples obtained from boiled unfrozen vegetable. A reverse dependence was observed for samples obtained from frozen broccoli. The water residue from unfrozen broccoli was characterized by the highest antioxidant activity.