Multicast is the multiparty communication that allows one-to-many communications. There are some multicast applications which are the ways of multiparty communications such as video conference system and white board. This paper proposes a dynamic allocation mechanism of designated receiver by using designated receiver list and a retransmission mechanism with designated receiver for the effective multicasting dta transmission. We designed voice conference system with multicast based on RTP/RTCP. This voice conference system increases the effect of standardization work and enables to exchange reliable standard dta with real time.