The article examines the Cossak issue in Siberia in the late XIXth – early XXth centuries. The author establishes that the development of capitalism in the agrarian sector of Siberian economy in the second half of the XIXth sentury involved priveledged Siberian Cossak army in commercial relations. Besides, the development of capitalism in the agrarian sector of Siberian economy evoked transformation of the estate structure of the Cossaks into the class structure. The main issue of the transformation was agrarian that led to the internal and external crisis of the Cossak army in Siberia.