Introduction: The “Six Ethical Codes for Research” in Iran includes ethical guidelines for “Genetic researches”, “Research on vulnerable groups”, “Research on animals”, “Research on gametes and embryos”, “Organ and tissue transplantation research” and “Clinical trials”. The policy-making council of the Ministry of Health and Medical Education and Tehran University of Medical Sciences’ Medical Ethics and History of Medicine Research Center prepared the codes in 2006. The purpose of these codes is to lay down moral guidelines for conducting research in the new aforementioned medical and biotechnological fields that have given rise to ethical and legal controversies. However, certain ambiguities and deficiencies seem to exist in these codes. This paper is a critical study of the “Six Ethical Codes for Research”. Conclusion: A desirable ethical code is supposed to embody a general, consistent and justifiable moral theory in accordance with which precise and clear guidelines are set out for relevant problems. The “Six Codes” does not represent such a theory and, hence, lacks clarity, compre-hensiveness and precision. Moreover, certain parts of the codes deal with non-moral (e.g. legal) and scientific (Technical) issues which definitely exceed the arbitrary ethical line.