A mobile Internet Protocol (IP) system and a data routing method of the mobile IP system are disclosed. According to the present invention, even if a mobile node moves from a first foreign agent to a second foreign agent while it receives data packets from the first foreign agent, the first foreign agent keeps storing the data packets it its buffer, and it sends the stored data to the second foreign agent indicated in a notification message received from the mobile node. Therefore, the host does not need to retransmit the data packets to the second foreign agent again. This will greatly increase reliability of the data transmission in the IP system and can reduce the overhead of the whole system.