The IP-based audio conference system is research and implemented in this thesis. The audio conference system is based on the SIP and RTP protocol. The centralized architecture is employed to design the conference server. All of IP-Phone, web-call, VoIP gateway, and soft-phone can easily access the service of audio-conference. The main contributions of this thesis are listed below: 1. The conference will work smoothly when user joins in and exits in any time. 2. The conference system accepts all of the ITU-based G series codec. 3. The conference system accepts different packet length.