Having no active mobility, anti-herbivore defense is an important determinant of plants' individual fitness. According to a concept of optimization, the defensive traits become monomorphic among individuals in a plant population, which maximize the individual's fitness. However, it has been reported that the polymorphism in defense level often exists within a single plant population. In order to explain such polymorphism, we consider an cooperative interaction among plant individuals in the defensive process called associational resistance, under which the one individual's defense reduce the neighboring herbivore pressure. We constructed a theoretical model in order to examine the evolutionary process of defense level under associational resistance, based on evolutionary dynamic theory. The result of analysis showed that the evolution of defense level strongly depending on the functional form of trade-off between defense and growth. When the functional form have inflection points, the dimorphic defense level can appear under the associational effect. We also examined the influence of spatial structure, fluctuation of environment by using computer simulations, but dimorphic defense level can evolve robustly. We concluded that associational resistance can promote the evolution of dimorphic defense level.