Mobile agents are autonomous objects that can migrate around a computer network. In addition to autonomy and social ability, mobility is the unique characteristic of mobile agents. Our objective is to model mobile agent systems from the perspective of system's architecture, using high level Petri nets as the formal basis. Underlying our research, agent migration and logical (strong) mobility is differentiated from physical mobility of hosts in ad hoc networks. The interoperability of mobile agent systems (which has been viewed as an important goal of mobile agent technology between various manufactures agent systems), and security policies of agent transfer and resource access are two other major concerns.