This paper analyses the performance of a nonblocking copy network for priority handling in a modular structured ATM multicast switch. Priority handling is required for a network congestion control as well as time critical services like video and voice conferences and interactive data services. The architecture of the non-blocking copy network is presented. The performance analysis for the copy network using an analytical method and a simulation are performed for both priority and non-priority cells. The results of the analysis in terms of a cell delay, a delay jitter, and a cell loss probability with different buffer sizes and fanouts under various input traffic loads are given in the paper. The results show that the proposed copy network provides a good performance for priority cells with a small sacrifice of non-priority cells.