Abstract: Ketol-acid reductoisomerase(KARI) is a promising target for the design of herbicides,yet there are only few literatures about the molecular design of KARI inhibitors. Based on the reported 0.165 nm high resolution crystal structure of spinach KARI complex,we obtained 279 molecules with low binding energy toward KARI from MDL/ACD 3D database searching with program DOCK 4.0,from which some compounds have been synthesized.Among 17 compounds,some showed favourable herbicidal activities on rape (inhibition rate 70.8 %) and barnygrass (inhibition rate 48.8 %) in greenhouse tests.