Cross　beam　is　one　of　the　major　parts　of　a　modern　CNC　machine　tool,　to　which　it　is　of　great　importance.　With　a　long　structure,　it　bears　the　weight　of　ram　and　slide　carriage.　Its　mechanical　structure　has　direct　influence　on　the　dynamic　performance　and　processing　accuracy　of　a　machine　tool.　This　paper　studied　a　gantry　type　cross　beam　which　was　composed　of　many　ribs　whose　good　structure　design　is　very　essential.　So,　in　this　paper,　the　cross　beam　and　the　ribs　are　the　major　research　objects　which　were　studied　by　the　finite　element　method　to　analyse　the　influence　of　cross　beams　with　different　rib　structures　on　the　processing　performance　including　dynamic　and　static　characteristic　of　machine　tool　to　find　out　a　better　beam　structure.