XML has become the most important technique to exchange data in WWW. Providing efficient access to XML document databases is thus crucial. Concurrency control protocols allow transactions to be executed concurrently to improve performance. However, no proposed research aims at the concurrency control protocol for XML databases. In this paper, we propose a lock-based concurrency control protocol, called XLP, for XML documents. XLP takes into consideration the access behavior of the XPath model, which is an important language for addressing data in XML documents suggested by W3C. XLP has the features of richer lock modes, refined lock granularity, lower lock conflict and lock conversion. We prove that XLP always generates serializable schedules. Our simulation results show that, for XML documents of different sizes, XLP outperforms 2PL by 84.4% on average and outperforms the tree locking protocol by 95.6% on average. The performance comparison of XLP with both 2PL and the tree locking protocol is also made in this study for various transactions with different XPath lengths, read/write ratios and whether the existence of predicates.