Honesty, fairness, respect for persons, avoiding conflict of interest, and keeping promises are ethical values accepted by almost all theories of business ethics. Nevertheless, more and more studies on cultural environment and business ethics have shown that cultural differences have a significant impact on cognition and behavior in business ethics. In this paper, we attempt to explore the hypothesis that post-modernism underlies the phenomena of cultural diversity. It is well recognized that post-modernism is an ever-changing, ever-present phenomenon. Its emphasis is on utilitarianism and practicality. It misunderstands pluralism and promotes value-neutrality. We try to respond to the challenges posed to business ethics theories by post-modernism thinking through an in-depth analysis of four cases, and thus bring to light the inconsistency of cultural diversity and moral relativism. We demonstrate that post-modernist thought is the real force behind cultural diversity, that it results in moral relativism and inconsistency of ethical decision making, and that it is incapable of providing a practical framework for managers facing a moral dilemma.