The purpose of this study is to find the rationality value as seen in Sophie Amundsen's character and to know how that rationality value defines her character. This study is categorized as a descriptive qualitative design. The source of data is taken from the Sophie's World novel by Jostein Gaarder. This study used rationality theory from the rationalist such as Socrates, Plato, and Descartes, with the addition from Suwardi Endraswara and Ranjabar. The result of this research showed that the rationality values found in Sophie's World novel are reasoning, doubting, and finding the agreement. The most appear rationality value is Sophie's reasoning. When Sophie accept her lesson about philosophy, she shows her reasoning in case of receive her philosophical lesson. From her reasoning, Sophie's character changed. Philosophy taught her about thinking rationally, logically, systematic, and critical. Thus, we can see that Sophie Amundsen is a dynamic character. From those three rationality values, Sophie Amundsen becomes thoughtful, skeptic, and has an understanding to others opinion that make her become wiser.