Efficient service discovery in dynamic, crossorganizational is one of the challenge aspects in ChinaGrid. Network overlay and search algorithms are two important considerations to the problem. Tree topology of organizations is easily managed but the root is a single point of failure; P2P structure tends to be conversed. To merge the advantages of both, we present a hybrid system with two layers: Tree Layer and Peer Layer. This structure is practical because organizations targeting at sub objects of a subject are inclined to be organized hierarchically as a Superpeer; and all these Superpeers construct an unstructured P2P network that adapts to peer?s interest by Learn Neighbor Algorithm. Experimental evaluation shows that our mechanism exhibits better search performance than fully hierarchical or fully distributed system.