The present article is showing and describing the activity of Oskar Halecki - a professor of the University of Warsaw - who was one of the most famous Polish historians of the 20th century, and who was known in public as a secretary of the League of Nations Committee on Intellectual Cooperation. Up till now the issue has not been the subject of thorough historical researches. The paper was prepared by dint of the analysis of source materials mainly from the Record Office in Geneva, Registry of UNESCO in Paris and the Manuscripts' Department of the Library of the Warsaw University. O. Halecki was a votary of the League of Nations and marked this fact already during the peace conference in Paris as an expert of Polish trip. In 1921, having been recommended by Inazo Nitobe - a general under-secretary of the League of Nations, a thirty-year-old professor Halecki was employed in Secretariat of organization. Having established in 1922 the League of Nations Committee on Intellectual Cooperation, which consisted of twelve eminent scholars from different countries, Halecki was appointed its secretary and acted till March 31, 1924. Very fast he found approval both of the League of Nations Committee's and Secretariat's members, especially thanks to his extreme activity, invention and keenness in organizing works of the Committee for the development of international intellectual cooperation. His great trump was also the fact he spoke several foreign languages. Halecki also backed up the idea of incorporating in international intellectual cooperation the new countries of East and Central Europe, and helping their scientific institutions. Halecki felt peculiarly obliged to Polish science. He wanted to ensure for Poland an adequate representation and participation in intellectual cooperation within the League of Nations. Thanks to his efforts Maria Sklodowska-Curie - a honorary professor of the University of Warsaw - became one of the members of the Committee as a representative of Polish science. Being the member of the Committee Sklodowska-Curie could count on permanent and varied help of Halecki, who paid much attention to establishing and activity of Polish 'national committee' on intellectual cooperation as a kind of the 'bond' both with the League of Nations Committee on Intellectual Cooperation and international intellectual life.