The paper aims at distinguishing the assumptions and component parts of financial system sustainability formation. Partly, sustainability of financial system can be expressed through the functions of financial system. Three financial subsystems are distinguished in the research: public finance, business finance and personal finance. The sustainable and efficient operation of each subsystem contributes to the sustainability of financial system as a whole. Also, sustainability of each of the subsystem can be measured by different indicators. In order to determine the strength of impact of various financial functions to the financial system such indicators as capital investments into financial and insurance activities, financial and insurance activities’ value added, as well as value of production and purchase of goods and services by the financial companies is analysed. Finally, the scheme of financial system sustainability is presented. The key conclusion of the research states that the synergistic effect of sustainable development of three fields of finance influences the sustainable development of the whole financial system and even can spread its impact beyond the limits of financial system.