Abstract: To model the uncertain actions of virtual characters is helpful to improve the fidelity and credibility of its behavioral representation.I3ecause the existing behavioral models are lack of necessary uncertainty, the characters often behave in a repeated and monotonous way. With probability interval, a method to model uncertain results of characters'action was proposed. Based on interval algebra, the concepts of probability interval and its completion were defined.Completion methods of probability interval were given. centering on normal probability interval, some operations were defined, and additional and multiplicative compensation methods to complete normal probability interval were discussed.Compared with algebra interval, probability interval can describe effects of actions more precisely. As an example, the uncertain actions of virtual tanks were discussed and normal interval was applied to model the uncertain actions of it.The proposed methods were validated through modeling the uncertain actions such as shooting.

Key words: Uncertain action,Probability interval, Virtual reality,Behavior modeling