Multicasting functionality is an important criterion in judging the powerfulness and extendibility of ATM switches. Recently many multicast ATM switches have been proposed. They are mostly modified from unicasting switches. For such switches to support multicasting, they often need some additional components, e. g. , copy network, and numerous modifications on the original unicasting network. Although the multicasting is supported, the method is often inefficient and the architecture is complicated and costly. In this paper, we propose a new multicast ATM switch. In the proposed architecture, the nonblocking unicast network will not be modified. All that need is a simple external nonblocking multicast module. The external nonblocking multicast module will not delay the original nonblocking unicast network module, and the cost in hardware complexity for the external nonblocking multicasting module will be O(NK log K), where N is the switch size and K is the number of broadcast buses used in the multicast module.