The larvicidal activity of materials from Ostericum koreanum (Apiaceae) root against late 3rd or early 4th instar larvae of Aedes aegypti and Culex pipiens pallens was examined using standard WHO technique under laboratory conditions. Results were compared with those of temephos. The biologically active constituents of O. koreanum root were characterized as four coumarines; isoimperatorin (I), osthol (II), oxypeucedanin (III) and imperatorin (IV) by spectroscopic analysis, including EI-MS, LC-MS, 1H NMR and 13C NMR spectroscopy. On the basis of 24-hr LC50 values, osthol among these four coumarines was the most toxic against Ae. aegypti (1.97 ppm) and Cx. pipiens pallens (1.87 ppm) larvae, but its activity was less than temephos (0.0047 and 0.013 ppm, respectively). LC50 values of isoimperatorin, oxypeucedanin and imperatorin derived from O. koreanum root against Ae. aegypti larvae were 2.46, 6.92 and 2.17 ppm, and against Cx. pipiens pallens larvae, were 1.99, 5.04 and 3.68 ppm. The results indicate that both materials and coumarines from O. koreanum root have potential to be used for mosquito control.