Insecticidal activities and active ingredients of Stephania kwangsiensis were studied for the first time. The results showed that all parts of Stephania kwangsiensis had contact activity to brown planthopper, Nilaparvata lugens, and the contact activity of methanol extract from tuber was highest, with a LD50 value being 1.5794 ?g/female. l-roemerine was isolated from tuber of Stephania kwangsiensis and identified, and it was the main active ingredient. l-roemerine had high contact toxicity to brown planthopper, with a LD50 value being 0.0443 ?g/female. Contact toxicity of l-roemerine to brown planthopper was 7.48 times that of malathion, the convientional chemical insecticide used for controlling brown planthopper. l-roemerine had also stomach poison activity to brown planthopper.