In this paper, we propose a dynamic multicast routing algorithm based on probability for multimedia communications in an asynchronous transfer mode (ATM) network environment. For multimedia communications, we not only aim to arrange a transmission path with the minimal cost from a source node to each destination node, but also consider the dynamic multicast situation with node joining/leaving. To meet the above requirements, we develop a new method based on the relative probability of each node being truncated from the multicast connection tree, such that we can arrange a better path for a new joining node in such a way as to reduce the impact of leaving requests caused by other nodes in the multicast connection tree. The main contribution of this paper is a new and efficient dynamic multicast routing algorithm for multimedia communications which keeps the whole multicast connection tree near optimal in terms of cost inefficiency and guarantees that the transmission delay will be less than the preset bounded end-to-end delay. Simulation results show that under various sizes of networks and requests, the cost inefficiency of the multicast connection tree constructed by our approach is less than those of the trees constructed by the greedy algorithm and the weighted greedy algorithm, respectively.