To better understand the biological properties of cupuassu seed residue resulting from fruit industrial processing, its antioxidant activity was determined by both the 2,2-diphenyl-1-picrylhydrazyl and 2,2 '-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) assays. Molecular modeling was used to correlate the antiradical power of eleven known cupuassu phenolic compounds with their structures. The extract antioxidant activity by both methods was very high compared with those of similar residues. The properties of the highest occupied and lowest unoccupied molecular orbitals indicate that the most energetically stable compounds possess the highest antioxidant activity. This was confirmed through the molecular electrostatic potential map by the nucleophilic surfaces on the oxygen atoms of carbonyl and hydroxyl groups.