Abstract: Actuality of the problem to be investigated depends on the circumstances that the armed conflicts have become widespread in the modern world. To settle up them the UNO carries out the peacemaking operations and the study of the process of formatting the peacemaking operation concept provides today an optimal policy of its fulfillment. The study aims at investigating the formation process of the concept of the peacemaking operations and the role of the Secretary General of the United Nations D. Hammarskjold there. The study deals with the formation process of the concept of peacemaking operations major contribution to which was made by the second UN Secretary General Dag Hammarskjold who was holding the post in the period of the years 1953-1961. The researcher considers, the attempts of forming international armed forces for the response to threatening to international peace and security. By way of the example of settlement of the conflicts in the Middle-East and the Congo the basic principles of peacemaking concept of Hammarskjöld are stated. The materials of the study may be useful in the process of discussion of a possible concept reformation of peacemaking operations of the UNO.