Although, intuitive interface signs are crucial elements of a good user interface (UI), prior research ignored these in UI design and usability evaluation process. This paper argues that ontology (the set of concepts and skills for understanding the referential meaning of an interface sign) mapping is critical for intuitive sign design. An experiment with fourteen participants and twelve signs (total 168 cases) has been carried out. The data has been analysed using both qualitative and quantitative approaches. The findings suggest that ontology mapping is important for understanding the semantics of interface signs. The paper concludes with design implications and future research plan.