We isolated a coumarin compound, isoimperatorin ($C_{16}H_{14}O_4$ mw: 270) from Angelica koreana through silica gel column chromatography, and characterized it by NMR. Here, for the first time we observed that isoimperatorin (25, 50 and 100 ${mu}M$) treatment for 24-72h inhibited growth and induced death in human prostate carcinoma DU145 cells. Further, in mechanistic investigation, isoimperatorin-induced cell growth inhibition was associated with a strong increase in G1 arrest in cell cycle progression, which started at 24h of the treatment. These findings suggest a novel anticancer efficacy of isoimperatorin mediated via induction of G1 arrest against hormone refractory human prostate carcinoma DU145 cells.