The traditional buffer replacement algorithms do not perform well in the multicast Video-on-Demand (VoD) systems. In this paper, we propose a new algorithm named Urgent Overlap Frequency (UOF) for merge-based multicast VoD servers. It makes good use of the merge property of the multicast channels. Unlike traditional ones that either consider time urgent degree or access frequency, three factors, i.e. time urgent degree, access overlap and access frequency, are considered in UOF algorithm. The experimental results show that UOF can increase the hit ratio 90 percent over LRU/MRU and 20 percent over BASIC algorithm in multicast circumstance. Additionally, the hit ratio of the UOF algorithm is only 4 percent lower than the theoretic optimal result, so it is very suitable for merge-based multicast VoD system.