The Behaviors of Intelligent Virtual Agents displayed in Virtual Reality should be derived from theirs "intelligence". So we extend ConGolog to EConGolog by formalizing the belief and intention of agents under the framework of Situation Calculus and adding communication actions in the earlier ConGolog. Meanwhile, we propose a novel cooperation model of Requirement/Service and corresponding cooperation Semantics for multi-agent system based on EConGolog. We give a case of the multi-agent system to achieve the cooperation based on reason about action and performance of action among agents in the dynamic and incompletely known environment. In addition, we develop an animation description language ECGADL based on EConGolog. It can implement the animation of various complex behaviors of the agents more conveniently and flexibly.