Semantic web makes the applications understand by the machines. Ontology is the building block of semantic web because it provides semantics to the information. OWL is the knowledge representation language for creating ontology and helps the system to automatically process and integrate the information. UML is unified modeling language which provides a set of graphical notation to create visual models. UML helps in designing and documenting a system and many enterprises have their schemas in UML. By using the existing UML models, OWL can be generated because similar properties exist between them. The class diagram plays a major role in conversion because both are based on classes, their relationships and instances.