Abstract: Type of structural units, degree of polymerization and mean molecular weight of condensed tannins extracted from calyx of large red mangrove (Bruguiera gymnorrhiza) were characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Crude cyanidin products were prepared from condensed tannins of B. gymnorrhiza by means of butanol/HCl reaction. In addition, the free radical-scavenging activity and antioxidant ability of crude cyanidin products were determined by using 1,1-diphenyl-2-picrylhydrazyl radical (DPPH·) and ferric reducing/antioxidant power (FRAP) model systems respectively. The results showed that catechin/epicatechin were the structural units occurring in condensed tannins of B. gymnorrhiza. Average degree of polymerization was 7.5, and mean molecular mass was 2081.60 u. Crude cyanidin products showed a very good DPPH radical scavenging activity (IC50 of 43.889 mg/L) and ferric reducing/antioxidant ability ((7.718±0.109)mmol/g).