Khajeh Nasiroddin Tusi’s ethical theory is one of the most important and noteworthy theories about Islamic philosophical ethics. In this paper we analyze his ethical theory from an ethical epistemology perspective−which is itself one of the main branches of meta-ethics. The primary issue in his theory is the possibility of ethical epistemology. According to the fact that he believes the good and evil actions are rational, it can be concluded that knowledge will make the ethical precepts possible correctly or incorrectly. The second issue is the method of justifying ethical precepts. According to the findings of the study, about justifying the ethical propositions, he is a naturalist in terms of definition. At the end, due to the significance of relativism in epistemology of ethics, absoluteness or relativity of Tusi’s ethical theory will be evaluated. Based on the main principles of his ethical theory it could be concluded that he rejects ethical relativism and accepts the generality and universality of ethical precepts