Abstract: The enantiomers of acetochlor and propisochlor were separated using high performance liquid chromatography with a Chiralpak AS-H column.The mobile phase was n-hexane with organic alcohols as polar modifiers.Absolute configurations of the enantiomers were identified by a combination of UV-Vis,Cirular Dichroism,ORD detector and the octant rule.The results showed that the first eluted peak for both acetochlor and propisochlor had the negative cotton effect and showed (-) ORD signal at 220 nm corresponded to aR-isomer.Baseline enantiomeric separation of acetochlor and propisochlor was achieved with mobile phase of n-hexane-isopropyl alcohol (97∶3 by volume).Moreover,the enantiomeric resolution was controlled by enthalpy term,in which the N-alkoxyalkyl substituents played important roles.