The advance of technology often requires the emergence of complementary technologies, of which the transition from IPv4 to IPv6 presents a significant example. The move of protocol has focused attention on the level of performance for associated technologies. Among the many Internet applications, in contemporary digital communications, VoIP stands apart in importance. This paper presents a performance analysis of VoIP using IPv4 and IPv6. Using OPNET to simulate the protocols and to investigate areas of performance weakness.