As　a　theoretical　foundation　of　relational　database,　integrity　constraints　such　as　functional　dependency　play　very　important　role　in　the　design　of　relational　data　schema.　Moreover,　the　wide　application　of　XML　data　requires　relevant　integrity　constraints　to　normalize　the　design　of　XML　documents　and　the　design　of　XML　database.　Because　XML　data　is　a　semi-structure　data,　we　find　that　data　redundancy　in　an　XML　document　results　not　only　from　functional　dependencies　between　the　data　values　of　different　data　items,　but　also　from　the　dependencies　between　the　structure　of　some　polymorphic　data　and　values　of　its　members　in　XML　Documents.　As　a　new　class　of　XML　constraint,　we　propose　the　concept　of　XML　type　dependency　(XTD),　to　indicate　the　type/value　dependencies　in　XML　documents.　We　give　the　formal　definitions　of　the　XML　type　dependency　and　its　instance　constraint　and　its　satisfaction　checking　algorithms.　We　also　propose　a　XTD-based　normal　form　and　a　normalization　algorithm　for　well-designed　XML　documents.　We　have　also　proved　that　the　redundancy　caused　by　the　XTDs　can　be　eliminated　by　this　normalization.　©　ACM　2016.