Discovery　of　web　services　has　gained　great　research　attention　due　to　great　numbers　of　web　services.　However,　many　functionally-equivalent　web　services　are　returned　by　semantic　web　services　registry.　So　it　is　necessary　to　rank　the　web　services　which　have　similar　functionality.　We　often　uses　Quality　of　Service　(QoS)　to　filter　the　semantic　web　services　discovery　results　based　on　user　constraints　of　QoS　descriptions.　But　different　service　providers　and　petitioners　may　use　different　QoS　concepts　and　measurement　methods　for　describing　service　quality.　This　leads　to　the　issues　of　semantic　interoperability　of　QoS.　In　this　paper,　we　firstly　analyze　the　classes,　attributes　and　relationships　from　QoS　vocabulary,　then　we　design　and　build　a　general　and　flexible　QoS　ontology　to　support　web　services　non-functional　requirements(NFRs).　Finally,　we　propose　a　semantic　web　services　discovery　algorithm　based　on　QoS　ontology.　The　algorithm　supports　the　automatic　discovery　of　web　services　and　it　can　improve　the　efficiency　for　users　to　find　the　best　services.　©　2010　Springer-Verlag.