The hit rate, a major metric for evaluating proxy caches, is mostly limited by the replacement strategy of proxy caches. However, in traditional proxy caches, the hit rate does not usually successfully predict how well a proxy cache will perform because the proxy cache counts any hit in its caching space which has many pages without useful information, so its replacement strategy fails to determine which pages to keep and which to release. The proxy cache efficiency can be measured more accurately using the valid hit rate introduced in this paper. An efficient replacement strategy based on the Site-Graph model for WWW (World-Wide Web) documents is also discussed in this paper. The model analyzes user access behavior as a basis for the replacement strategy. Simulation results demonstrate that the replacement strategy improves proxy cache efficiercy.