With the rapid development of Internet, traditional Client/Server model can not meet the need of the distributed computing which is based on low bandwidth and high delay Internet. The appearance of mobile agents, a new distributed computing model in which every host is peer but not traditional master/slave, is inevitable. However the implementation of current mobile agents system does not take much of the performance into account. Based on the analysis of the code migration mechanism of existing system such as (Visual) Obliq and Java Applets, CodeBroker-based mobile agents system sees the agent code as the set of named code( or classes ) and loads the code on need and transparent to code distribution . This code migration mechanism not only improves the performance and fault tolerance of mobile agents system but also facilitates the sharing of Internet resources . A brief view of the CodeBroker implementation shows that it is practical.