Moral relativism is the philosophical theory that morality is relative that different moral truths hold for different individuls or different societies. Individual and social differences and variety of needs in different ages has posed this question whether morality is relative or absolute? Although Kant and Mulla Sadra belong to different reflective systems but both have common problems in this topic. Both have founded moral laws upon "practical reason" to deliver them from relativism risk. In Kant, "moral law" and "pure practical reason" have universality, and human as "pure intelligence" and "tendency toward highest good" becomes eternal. In Mulla Sadra, human is a "comprehensive reality" that comprehends universal realities in speculative sphere and particular acts in practical sphere. Also on the basis of "Imagination immateriality" and "individual unity of soul", human and soul habits are eternal. In this article we critically analyze and compare viewpoints of Kant and Mulla Sadra about the problem of moral relativism.