Decentralized and unstructured peer-to-peer (P2P) networks such as Gnutella are attractive for large-scale information retrieval and search systems because of their scalability, fault-tolerance, and self-organizing nature. Because of this decentralized architecture, however, traditional P2P keyword search systems are difficult to globally share useful semantic knowledge among nodes. As a result, traditional P2P keyword search systems cannot support semantic search (support only naive text-match search). In this paper, we describe a design of the semantic P2P keyword search system. We exploit the semantics of correlation among keywords rather than synonym. The key mechanism is query expansion, where a received query is expanded based on keyword relationships. Keyword relationships are improved through search and retrieval processes and each relationship is shared among nodes holding similar data items. This semantic P2P search system has two main advantages. First, expanding search results through query expansion increases the possibility of locating desired data items which would not be found by traditional P2P search systems due to the keywords' textual mismatch. Second, keyword relationships originally introduced for query expansion, can be used for result ranking. Our main challenges are 1) managing keyword relationships in a fully decentralized manner and 2) maintaining the quality of search results, while suppressing result implosion. We also describe the prototype implementation and evaluation of the semantic P2P search system.