ABSTRACT IP multicast technology fits data transmission services of one-point to multipoint or multipoint to multipoint. The realization of multicast underlies the accomplishment of IP protocol. In multicast mechanism, no matter how many members in the group, data source sends packets only once. What’s more, only the computers needing data packets can receive them. In the shared link, the same information needs only one multicast stream. Therefore, it can control traffic well and decrease the load of computers and the network. Time server belongs to a one-point to multipoint data transmission. It requires servers to send accurate time to a group. So time server selects multicast technology. In this way the multicast terminal will be decreased. Meanwhile, as UDP protocol is applied in multicast, the Congestion in the network can not reduce the sending rate of source computer. Java API gives great backup to multicast, as well as the characteristics of Java itself, so we choose Java for the realization of time server. A simple client identification is added into this designing. First a client should send authentication data to the server. Only after receiving the identification message from the server can the client receive the multicast data sent by the server successfully. Key word: IP Multicast Time Server Java Network Programming