Abstract: 20 kinds of insecticides against 4-5 instar larva of Cnaphalocrocis medinalis were tested. The result showed that the best effect was abamectin, next was fipronil and endosulfan in 24h. The pyrethroid insecticides that included cyhalothrin, fenpropathrin, beta-cypermethrin and the bisultap could still control the Cnaphalocrocis medinalis. The effect of the organical phosphate insecticides that included triazophos, acephate, phoxim, chlorpyrifos, methamidophos, parathion-methyl, dichlorvos and mixture of chlorpyrifos+ abamectin, phoxim +triflumuron and phoxim +cypermethrin against older instar larva of Cnaphalocrocis medinalis were nothing. The reselt of the same insecticide against different popution instar larva of Cnaphalocrocis medinalis were difference.