Cranberries (Vaccinium macrocarpon) contain many bioactive compounds and have some biological activities and beneficial health properties. In the study, antioxidant effects of lyophilized aqueous extract of cranberry (LAEC) and quantity of some its polyphenolic compounds were determined. For this purpose, we performed DPPH., DMPD.+, ABTS.+ and O2.- radicals scavenging activities, inhibition of lipid peroxidation activity by thiocyanate method, Cu2+ and Fe3+ reducing abilities, FRAP assay and Fe2+ binding activity. At the 10 micro g/mL concentration, LAEC inhibited 52.4% lipid peroxidation produced by linoleic acid emulsion. Also, alpha -tocopherol, BHA, trolox, and BHT had 52.5, 89.9, 93.1 and 94.9% inhibition value at 30 micro g/mL concentration, respectively. Quantitative amounts of some phenolic compounds in LAEC were investigated by high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS). p-Hydroxy benzoic acid was found as the most abundant phenolic compound (55 mg/kg extract) in LAEC.