Hybrid Peer-to-Peer(P2P) systems that construct overlay networks structured among superpeers have great potential in that they can give the benefits such as scalability,search speed and network traffic,taking advantages of superpeer-based and the structured P2P systems.In this article,we enhance keyword search in hybrid P2P systems by constructing a tree-based index overlay among directory nodes that maintain indices,according to the load and popularity of a keyword.The mathematical analysis shows that the keyword search based on semi-structured P2P overlay can improve the search performance,reducing the message traffic and maintenance costs. Hybrid Peer-to-Peer (P2P) systems that construct overlay networks structured among superpeers have great potential in that they can give the benefits such as scalability, search speed and network traffic, taking advantages of superpeer-based and the structured P2P systems. this article, we enhance keyword search in hybrid P2P systems by constructing a tree-based index overlay among directory nodes that maintain indices, according to the load and popularity of a keyword. The mathematical analysis shows that the keyword search based on semi-structured P2P overlay can improve the search performance, reducing the message traffic and maintenance costs.