The aim of this thesis is to explain the role and the significance of Ante Trumbić during the formation of the Kingdom of Serbs, Croats and Slovens. It starts with a brief summary of political and social circumstances in the Southeast Europe during the late 19th and early 20th century, with an emphasis being made on the different on the Yugoslav idea, and the Croatian and Serbian national ideology. It continues With Trumbić's evolution as an important political figure - from his early political days influenced by the ideology of the Party of Rights through the "New Direction Policy" and Rijeka Resolution; through his days in the Yugoslav Committee and involvement in Corfu and Geneva Declaration, and finally the formation of the Kingdom of the Serbs, Croats and Slovenes. The first part of the thesis tries to explain the significance of Ante Trumbić's political activities in the Party of Rights in Dalmatia, which he had been leader of (together with Frano Supilo and Ivo Prodan). This is followed by the part about his work in the Diet od Dalmatia and the Imperial Council in Vienna, which underlines the importance of the "New Direction Policy" (shaped by Trumbić himself in the early 1900s) for the Croatian political stage of the time. Resolution of Rijeka is specially analyzed because it is a direct consequence of the "New Direction Policy" and because it was a new milestone for Croatian social and political elites. The second part of the thesis covers Trumbić's role in the creation of the Yugoslav state, most evidently noted in his work as a president of the Yugoslav Committee. Finally, this paper deals with Trumbić's relationship with Nikola Pašić and their clashes regarding the political system of the new state, and interpretations of Corfu and Geneva Declarations. Of course, all of this has to be put ( as it is, in the text) into the wider context of the main political events taking place on the global stage. Also, the roles of the world's political and social elites in the process of formation of the Yugoslav state - especially those of the Great Britain, USA, France, Russia and Italy - had to be explained.