We present in this paper an extended object model for software system modeling and design. The extended object model was developed from ontological research into the nature and the generality of intelligent systems. The extension was made by attaching domains, states and categories to variables, and adding three types of constraints into the ordinary object model: identity constraints are for maintaining the identity and integrity of objects; trigger constraints are for enabling agents to act in objects autonomously; and goal constraints are for guiding agents to act in desired direction. We first introduce the theoretical background of the object model in brief. We then present the models in detail. We also discuss the advantages of our extended object model in software system modeling and design. In conclusion we summarize the main results we have achieved, and discuss some ongoing works that are relevant.