Mobile agent computation will be the main paradigm of the future network computation. The mobility is the essence of mobile agent technology. After surveying some representative approaches to migration models used in mobile agent systems, the authors propose a new structured migration mechanism in this paper. In this mechanism, the itinerary and the function body of an agent are separated completely and the itinerary scheme itself has its own strictly defined structure. Besides, three kinds of migration mode are also provided. So the complexity of mobile agents can be controlled and the reusability is supported. This mechanism has been used in the mobile agent system Mogent 1.0.