Twenty-five compounds were isolated from the methanolic extract of Geum japonicum (Rosaceae), and their structures were identified as eleven triterpenoids [ursolic acid 3-acetate (2), cecropiacic acid 3-methyl ester (3), pomolic acid 3-acetate (5), ursonic acid (6), ursolic acid (7), pomolic acid (8), corosolic acid (9), euscaphic acid (11), arjunic acid (16), tormentic acid (18), 23-hydroxytormentic acid (21)], two saponins [rosamultin (22) and kaji-ichigoside $F_1$ (23)], two megastigmanes [blumenol A (14) and (+)-dehydrovomifoliol (15)], three flavonoids [apigenin (13), isoquercitrin (17) and tiliroside (24)], two ellagic acid derivatives [3,3'-di-O-methylellagic acid (12) and ducheside B (25)] and five others [eugenol (1), emodin (4), vanillic acid (10), gallic aldehyde (19), salidroside (20)]. The chemical structures of these compounds were identified on the basis of spectroscopic methods and comparison with literature values. This is the first report of the eleven compounds, 2~6, 10, 15, 16, 20, 23, and 25 from the genus Geum, as well as the first report of apigenin (13) and 3,3'-di-O-methylellagic acid (12) from G. japonicum. The antioxidant properties of 22 isolates (1~11, 14, 16~25) were evaluated by the intracellular reactive oxygen species (ROS) radical scavenging using 2',7'-dichlorodihydrofluorescein diacetate (DCF-DA) assay. Among them, isoquercitrin (17) showed significant scavenging activity, and gallic aldehyde (19) and ducheside B (25) showed weak scavenging activity.