It directly affects the functions and economic benefits of products to choose a fit of geometric product components. In order to meet these functions, save the costs and share the information passing during the entire life cycle of products, this paper constructs a priority fit ontology which refers to ISO 286 and the new generation of GPS. Concepts are extracted from the influencing fit factors, such as tolerance grades selection, fit types selection and fit systems selection. Classes are constructed with above concepts in Protégé. A few necessary properties are added to restrict classes of the ontology. Some individuals are created to facilitate the reasoning and query of the best fit. A fit case is shown to achieve the recommendation of the best fit with given information by the priority fit ontology. Because of the similarity between priority fit and other kinds of fits, the technical reference are provided to construct the ontology of common fit, general fit and any fit.