For a Peer-to-Peer (P2P) system which contains huge amount of data, an efficient content search technique is definitely necessary. In this paper, we present a novel category overlay search infrastructure which can sit on top of existing unstructured P2P systems, such as Gnutella. The intuition behind our work is that by clustering the whole peer group into clusters and mapping the predefined category domain to clusters, multiple category overlays can be constructed and a search can be restricted within only some specific overlays, thus the search efficiency can be improved dramatically. Our preliminary simulation shows that this infrastructure can provide efficient search service, even in a highly dynamic network environment.