Identification and measurement of similarity between web ontologies is a mandatory pre-requirement of various reuse operations of ontologies such as merging, mapping and alignment. It is also a mandatory requirement for engineering new ontologies by assembling exiting ontologies or components of ontologies.The proposed technique upgrades similarity measurement criteria, from terms to concepts, from linguistic semantic to explicit semantic and from all taxonomic characteristics of concepts to mandatory and optional characteristics. In addition we introduced the concepts of similarity levels: primary similarity or 1st level similarity, contextual similarity or 2nd level similarity and the role-based similarity or 3rd level similarity. Furthermore, a layered strategy has been adopted in proposed technique to enable matching process more efficient. New criteria have been introduced for measuring the degree of similarity and the semantic relations between concepts.