Three anthraquinones (1, 2 and 4), three stilbenes (5, 6 and 7) and 3,5-dihydroxybenzyl alcohol (3) were isolated from Reynoutria japonica. Their structures were identified as emodin (1), emodin-8-O-${eta}$-D-glucoside (2), 3,5-dihydroxybenzyl alcohol (3), citreorosein (4), cis-resveratrol (5), trans-resveratrol (6) and trans-resveratrol-5-O-${eta}$-D-glucopyranoside (7) by comparing their physicochemical and spectral data with published data. Compound 3 was isolated for the first time from the Polygonaceae family. Among the purified compounds, 3 showed more potent inhibitory activity against topoisomerase I ($IC_{50}:;4{mu}M$) than camptothecin, as the positive control ($IC_{50}:;18{mu}M$). Compounds 3, 4, 5, 6 and 7 showed stronger inhibitory activities toward DNA topoisomerase II ($IC_{50}$: 0.54, 14, 15, 0.77 and 3 ${mu}M$, respectively) than the positive control, etoposide ($IC_{50}:;44{mu}M$). Compounds 1 and 4 displayed weak cytotoxicities against human lung cancer (A549), ovarian cancer (SK-OV-3), human liver hepatoblastoma (HepG2) and colon adenocarcinoma (HT-29) cell lines.