Seven xanthones; dulcisxanthone B (1), ß-mangostin (2), 1,3,7-trihydroxy-2,4-di-(3-methylbut-2-eyl)-xanthone (3), cudratricusxanthone E (4), cochinchinone A (5), 2-geranyl-1,3,7-trihydroxy-4-(3,3-dimethylallyl)-xanthone (6), and cochinchinone B (7) were isolated from twigs of Cratoxylum cochinchinense and Cratoxylum formasum. The structures of all isolated compounds were elucidated by spectroscopic methods (1H, 13C, 1H-1H COSY, 1H-13C HSQC, and 1H-13C HMBC) and compared with previous literatures. All compounds were evaluated in vitro for antioxidant activity using DPPH assay. Their electrochemical behaviors were investigated by cyclic voltammetry. The results derived from two techniques were consistent. Compounds 1, 2, 4 and 7 exhibited strong antioxidant activities. Considering the structure-activity relationship, the hydroxy group at C-6 played an important role to antioxidant power.