New twenty five Imazethapyr derivatives, [2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-3-(N-methyl-N-X(sub.)-phenylaminooxoacetyl)-5-methylpyridine] were synthesized. and The quantitative structure activity relationships (QSARs) between their post-emergence herbicidal activity(pI_(50)) values in vivo against Barnyard grass (Echinochloa crus-galli) and physicochemical parameters of substituents(X) of 3-(N-methyl-N-X(sub.)-phenylaminooxoacetyl) group have been studied. From the basis on the findings, in case of post-emergence, the activities were dependent on the steric constant(E_s$lt;0) and electron donating (σ$lt;0) effect by subsitituents(X) of 3-(N-methyl-N-X(sub.)phenylaminooxoacetyl) group. Therefore, The most effective compound, 15 (4-t-butyl group) and 20 (3,5-dimethyl group) were examined in this study. And the conditions on the compounds predicted to show higher herbicidal activity were also discussed.