XQuery　is　a　powerful　functional　query　language　for　querying　a　broad　spectrum　of　XML　information　sources.　With　the　emergence　of　XML　as　the　de　facto　standard　for　data　representation　and　exchange　on　internet　in　recent　years,　the　data　volumes　and　querying　complexity　for　XML　data　are　growth　accordingly,　consequently　more　efficient　XQuery　processing　is　required.　When　knowing　invariants　in　program　before　executing,　program　specialization　can　potentially　improve　programs　executing　performance.　A　compiletime　specialization　for　XQuery　programs　has　been　realized,　but　the　dynamically　generated　queries　achieved　in　running　time　are　widely　adopted　in　realistic　XQuery　applications,　such　as　XQJ　interface　invoking　for　querying　XML　data　in　Java　programs.　So,　it　is　necessary　to　specialize　XQuery　programs　in　running　time.　In　this　paper,　we　present　a　run-time　specialization　technique　for　XQuery.　An　XQuery　runtime　specialization　system　has　been　implemented　and　successfully　used　to　realize　XQJ　implementations,　and　it　significantly　improves　the　performance　of　XQuery　programs.　©　2012　Binary　Information　Press.