The public institution and establishments specified in the Public Procurement Law numbered 4734 use the public procurement procedure indicated in this Law for construction works, service and product procurements. Various principles have been set forth in the Public Procurement Law numbered 4734 in order to ensure that the public procurement process can be conducted in accordance with the law. These principles have been adopted to ensure that the public service can be carried out in an efficient and effective manner. Confidentiality principle is one of these principles. The aim of the present study was to put forth what the confidentiality principle is from among the fundamental principles set forth in the Public Procure- ment Law which should be implemented during the procurement stage. The confidentiality principle is implemented in certain limited areas indicated in the Public Procurement Law. In principle, confidentiality and the prohibition to disclose the approximate cost, quotations, related information and documentation prevail during the procurement process. The necessity for the confidentiality of approximate cost is also under discussion. Various enforcements have been envisaged for those who breach the confidentiality principle. Keywords: Public Procurement Law, public tender, fundamental principles in the Public Procurement Law, confidentiality principle, approximate cost