For concurrency control of XML data, this paper proposes a new locking method based on XPath with a new data management model. Although increasing concurrency for XML data becomes an important issue in XML data management, few effective methods providing high concurrency of transactions working on the same XML document have been proposed so far. To overcome this problem, we propose a locking method that can achieve high concurrency while guaranteeing the serial- izability of transactions to the same XML document. In the proposed method, logical locks are set on XPath expressions used in transaction accesses, and conflicts that violate serializability are detected each time a read or a write access is requested by concurrent transactions. Efficient algorithms for detecting conflicts are also proposed in this paper. Since the proposed locking is at the level of precise data in XML documents, high concurrency for XML data can be achieved assuring serializability.