Non-profit organizations play a significant role in society because they operate at a time when the state and the market cannot meet all social needs. They stimulate the growth and development of society and seek to satisfy the interests of both individuals and the public without the intention of making a profit. There is an interest in analyzing non-profit organizations financial sustainability, which this paper provides through analyzing their financial statements with a focus on accounting itself, because they play such an important role in society. The subject of the analysis are two non-profit organizations, the Association for Nature, Environment and Sustainable Development „Sunce“ and the Association SOS „Dječje selo“ Croatia. Based on the financial statements of the aforementioned organizations, a vertical and horizontal analysis is carried out, the selected indicators of non-profit organizations are calculated, and then, based on the obtained results, a conclusion is reached on the sustainability of their financing. The question arises as to whether the revenues generated from the own sources of the above mentioned organizations are sufficient for their financial sustainability, whether they raise the funds appropriately, and how much benefit they actually bring to the community. The paper provides a theoretical introduction to the conceptual framework, types, characteristics and specificities of non-profit organizations, as well as the regulatory framework within which they operate, detailing the characteristics of accounting and the principles of the financial management system. A key part relates to the analysis of the accounting of financing of these two selected organizations, where through a basic horizontal and vertical analysis of the Revenues and expenditure statement, as well as the Balance sheet, the sustainability of the financing of the observed organizations is assessed using additionally selected indicators. The main hypothesis of this paper is confirmed, namely, that the revenues generated from the own sources of the selected analyzed non-profit organizations are not sufficient for their financial viability.