The present study seeks to examine the effect of the theological rule of “rational beauty and ugliness” on “moral opinions” in the field of philosophical ethics by exploring the two books “Purification of Belief” and “Nasirean Ethics” written by the famous scientist, Khajeh Nasir al-Din Tusi. This research has been done by content analysis method and its findings show that on the one hand, the theological rule of rational beauty and ugliness revealed in “Purification of Belief” is based on an important part of theological issues related to origin, prophecy and resurrection. On the other hand, many of the moral opinions expressed in “Nasirean Ethics” are based on the same rule. The necessity of explaining this research is that the proof of the rational beauty and ugliness, and that many genuine moral values in individual and social issues are rooted in the understanding of reason and accepted by the wise of the world, provides a fundamental basis for moral virtues that obliges even the deniers of the divine laws to observe many moral principles.