To understand user behavior and usage pattern is critical for not only the deployment, such as load balancing and development, but also the research and simulation of new technology. In this research, we traced and characterized the usage pattern of mobile IP in campus network. The data, which have generated by 1567 users from 228 access points and home agents in 85 buildings, have collected since March 2003. The research result showed the difference of the usage patterns of students, professors as well as the increasing rate of users and sessions. We also analyzed daily usage, roaming pattern, session length and error rate which are different from normal usage pattern of wireless LAN. The result also shows that the users with frequent roaming are the professors, and the student users are mainly staying within one or two places despite of a rapid growing rate in usage.