In　order　to　supply　connection　moment-rotation　relationship　curves　required　in　the　analysis　and　design　of　semi-rigid　connected　structures,　beam-to-column　extended　endplate　connections　were　analyzed　by　using　a　finite　element　method.　Firstly,　a　finite　element　model　was　set　up　by　using　a　large-scale　general　purpose　code,　ANSYS.　Material　nonlinearity,　geometric　large　deformation,　contact-friction　behavior　and　bolt　pretension　were　taken　into　account.　It　is　noted　that　the　bolts　in　connection　were　modeled　with　using　a　proposed　model　verified　by　the　refined　finite　element　analysis　in　which　the　complex　geometries　of　threads　were　realistically　modeled.　The　accuracy　of　finite　element　model　was　verified　by　the　comparison　of　the　numerical　and　experimental　results.　Then　the　behavior　of　the　stiffened　and　the　un-stiffened　endplate　connections　under　the　bending　moment,　were　analyzed　by　using　the　aforementioned　finite　element　model.　Influences　of　some　factors,　such　as,　beam　depth,　endplate　thickness,　stiffer　for　endplate,　bolts　grade　and　diameters,　piths　of　bolt　holes,　thickness　of　column　flange,　on　the　connections　rotational　stiffness,　flexural　resistance　and　ductility,　were　discussed　in　detail.　Finally,　based　on　the　analysis　results,　moment-rotation　relationship　prediction　equations　for　the　un-stiffened　and　stiffened　endplate　connection　behavior　were　developed.　Meanwhile,　the　prediction　equations　were　validated　by　comparison　between　experimental　and　finite　element　analysis　results.