This study aims to identify the meaning and scope of Public Morality Principle. Using the deductive method of presentation of the study, it starts by drawing a picture on the theoretical framework of positivism, through post-positivism and ending in me- thodic-axiological positivism by Heleno Torres. Deals with the influence of private law in public law. Addresses the need for control of judicial decisions and the principle of Legal Security. Following analyzes the principle of morality relating it on justice, equality and legality. Then addresses the principle of legitimate trust and finally, comes the good faith of the public administration, again related to private law, and concludes with aspects of tax law interpretation, the moral of the tax authorities and the taxpayer.