This paper proposes a novel approach to generating an ontological perspective for software system. This ontology is a combination of two other ontologies: domain ontology and class diagram ontology. Domain ontology is domain vocabulary which is built by domain experts, while class diagram ontology is automatically populated from source code to represent the knowledge in the code. Combining these two ontologies will generate a third ontology which is used in program comprehension. Firstly, the motivation and related work are introduced. Secondly, method of class diagram to ontology transformation and algorithm of ontology combination are presented. Thirdly, program comprehension from an ontological point of view is discussed. Fourthly, a case study is demonstrated by understanding a Point of Sale Terminal (POST) system. Finally, the conclusion is drawn and further research directions are speculated.