Knowledge sharing of radiotherapy risk contributes to keep the safety practice between the oncologists, medical physicists, and therapists. But the mechanisms of risks tend to complex because of its diversities. Recently, an ontological analysis is tried to share knowledge in the various domains. Therefore, the aim of this study is to construct Radiotherapy Risk Ontology (RRO) and clarify the implicit structures of risks towards knowledge sharing. The method of this study is below. First, the classes and relations were extracted from the risk categorization of the publications. Second, these classes and relationships were connecting and mapping by the ontology editor. Third, the total relationships of classes were verified using paired comparison chart. This paper distinguishes between publication-oriented relationships and the other relationships. And these relationships was defined the explicit and implicit relationships. RRO was constructed 789 classes and 14 types of the explicit relationships. The 22 types of the implicit relationships were clarified using paired comparison. RRO provided the conceptual mapping and the implicit knowledge. The result of this study assisted the knowledge sharing.