The extracts of two native berries from the Jeju Island of Korea were used to assess polyphenolics and flavonol levels, superoxide dismutase (SOD)-like enzyme activity, free radical scavenging activity, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide cell viability, protective effects against H2O2-induced cow pulmonary artery endothelium (CPAE) cell cytotoxicity and inhibition of low density lipoprotein (LDL) peroxidation. The total content of the flavonols and phenolic compounds in the leaves was higher than the fruit for both berries. In the free radical scavenging and SOD-like enzyme assay, all extracts showed strong antioxidative effects. The CPAE viability was greatly reduced and the survival rate was near 37% when the cells were treated with 1.0 mM H2O2 for 24 h. The extracts also inhibited lipid peroxidation in human LDL, measured as reduction in malondialdehyde production. The results showed that the leaf extracts of Empetrum nigrum significantly protected human LDL from Cu2+-induced oxidation. E. nigrum is suggested as an excellent potential source of natural antioxidants.