With　the　fast　development　of　web　services,it　is　necessary　to　distinguish　and　rank　web　services　which　have　similar　functionality.　QoS　becomes　the　main　factor　to　distinguish　those　services.But　different　providers　and　participants　may　use　different　QoS　concepts　and　measurement　to　describe　service　quality,so　we　build　a　QoS　ontology　to　reduce　the　ambiguity　of　semantics　and　support　discovery　and　composition　of　web　services.　In　this　paper,　we　analyse　classes,　attributes　and　relationships　of　QoS　vocabulary　and　propose　a　QoS　model,　then　we　build　a　generic　and　flexible　ontology.Finally　we　give　a　semantic　web　services　composition　framework　based　on　QoS　ontology.　©　2010　IEEE.