Abstract: In order to evaluate the interaction of graminicide and broadleaf herbicide correctly, the interaction between quizalofop-p-ethyl and bentazon had been studied as an example. In the study isobole had been improved, and single axial isobole and the calculating method of synergic times had been advanced. The tested result showed that quizalofop-p-ethyl mixed with bentazon possessed synergism on Digitaria sanguinalis and Amaranthus retroflexs. Their average synergic times were 0.42 and 0.34,respectively.The interaction of graminicide and broadleaf herbicide could be correctly evaluated by single axial isobole.