Abstract: The traditional multicast switch fabrics (MSF) are mainly developed from the unicast switch fabrics (USF). Owing to the bottleneck of the current USF, almost all the existing MSF are not able to achieve high efficiency orexcellentlarge-scale scalability. A wire-speed MSF based on lattice theory is presented. It cannot only meet the requirements of superior scalability and wire-speed multicast, and has low delay, jitter free, and no queuing cache performance advantages. It is proved that the MSF can both achieve optimal multicast performance and support priority-based multicast which can guarantee the multicast QoS (quality of service) in delay.