Abstract: This article is dedicated to the questions of party building of the constitutional democrats of Siberia during 1917 – 1920, which is one of most tragic periods of the Russian history. Based on the analysis of  published and archival materials and documents of the central and local committees of the People's Freedom Party in Eastern Russia during the Revolution and Civil War, the goal is set to determine the causes and follow the trends of changes in composition of the party, vector and nature of the activity of Siberian cadet groups. The research relies on the fundamental principles of scientific objectivism and historicism, which allows examining the topic in dynamics and in relation to the specific socioeconomic, political, and cultural circumstances that developed during this period. The scientific novelty consists in demonstration of the dynamics and identification of the causes of changes in composition and tactics of the Siberian cadet groups throughout the Revolution of 1917 and the Civil War. Emphasis is placed on the “democratic counter-revolution” and the political regime of A. V. Kolchak. The conclusion is made that the nature of organizational party activity of the Siberian cadets is determined by a range of factors, namely local conditions and events that unfolded in the capital of Russia. A considerable influence was produced by the directives of the Central Committee of the People's Freedom Party, political orientations of the All-Russian National Center, and the experience acquired by the party and its leaders during the Revolution of 1917 and the Civil War.