We study the validation of XML documents when they are updated in XML databases. An XML document can be verified by checking against an XML Schema, which contains structure and type information of XML documents. However, most of XML database systems just validate the whole XML document, but can not validate parts of it. If updates are very frequent, then validating the whole XML document will cause serious performance degradation. Furthermore, rollback should be performed if the updates result in an invalid document, because the updated document is usually validated after the update operation executed. In this paper, we propose an immediate and partial validation mechanism for solving these two problems, i.e the validity of an update operation is checked immediately before the actual update is applied to the database whether it causes invalidity, and validation is performed only on the updated parts of the XML document in the database. Consequently, XML database systems can maintain valid XML documents at any time. We already proposed an immediate and partial validation mechanism based on DTD[6], and we extend the mechanism based on XML Schema in this paper.