This article argues that moral relativism is founded methdologically not on the irrationalism as usually thought,but on the rationalism.Modern moral rclativism began with rationalism of enlightenment.Both moral universalism and moral relativism take ratonality as their principle,but their understandings are differently.Moral relativism thinks that rationality has quite different meanings without any common points.It is as that all different peoples have common logical rationality that they have common praxis rationality on which the universal moral principle is founded.Just as contradiction to basic logical principles will lead to difficultics in life,any behaviors that contradict to praxis rationality will lead to difficulties in understanding to each other,and lead to negation of moral relativism.