H4　mechanism　corresponds　to　a　group　of　parallel　mechanisms　with　good　dynamical　character,　the　center　components　of　their　movable　platforms　have　three　translational　DOFs　and　one　rotational　DOF.　In　this　group　of　mechanisms　there　are　four　isomorphic　sub-chain　mechanisms,　each　sub-chain　includes　one　(S-S)2　quadrilateral　bar-suit,　and　the　difference　of　these　four　mechanisms　is　in　that　their　actuated　joints　or　the　sub-chains＇deployment　between　base　and　movable　platforms　are　different.　Based　on　the　structure　analysis　of　isomorphic　sub-chain　mechanisms,　the　force　equilibrium　equations　of　movable　platform　and　sub-chain　are　established,　and　force　Jacobian　matrix　are　obtained.　On　which　the　singularities　of　the　four　mechanisms　are　analyzed　by　using　line　geometry　method,　the　discriminating　conditions　of　sub-chain　singularity,　platform　singularity　and　actuator　singularity　are　proposed,　and　the　kinematic　characters　of　mechanisms　at　singularity　configurations　are　analyzed.　Finally,　the　configuration　examples　of　an　isomorphic　sub-chain　parallel　mechanism　in　the　states　of　sub-chain　singularity,　platform　singularity　and　actuator　singularity　are　illustrated.　©2010　Journal　of　Mechanical　Engineering.