Abstract: This article discusses an ontology of generic functional concepts of artifacts, called a functional ontology. We discuss the requirements for the ontology and the characteristics of some existing functional modeling languages. We present a functional ontology described by our functional modeling language FBRL in terms of its mapping primitives between behavior and function. They make the definitions independent of their implementation. The functional ontology enables us to realize a functional understanding system which identifies functional structures of an artifact from its behavioral and structural model. It plays a crucial role to specify and limit the functional space. This article also presents a framework of the functional understanding system and an example of functional understanding of a power plant.