Subject. This article focuses on the issues of improving the practical aspects of assessing the sustainable development of an organization, taking into account the relationship between external and internal factors and their compliance with the criteria of long-term sustainability. Objectives. The article aims to develop methodological approaches to assessing the sustainable development of an organization on the basis of identifying the relationship between internal and external factors of stability of an economic entity to determine key indicators affecting the types of capital. Methods. For the study, we used generalization, comparison, analysis and synthesis, a systems approach, and questionnaire. Results. The article describes the relationship between the factors of the business environment and the elements of financial, human and natural capitals of an economic entity, which is the basis for the identification of priority assessment indicators in the context of managing the sustainable development of an organization. It proposes to improve the methodology for assessing the level of sustainability of the projects being implemented by an economic entity based on the calculation of a complex indicator demonstrating the expected contribution of each project to the financial, human and natural capitals of the organization. Conclusions and Relevance. The proposed methodology for evaluating the projects being implemented from the standpoint of their contribution to financial, human and natural capitals on the basis of the calculation of a complex indicator establishes the impact of projects on the change in the level of stability of the organization. The information obtained at the stage of assessing the sustainability of the projects being implemented contributes to the development of mechanisms for managing the level of development of an economic entity. The results of the study can serve as a substantiation of organizational recommendations for managing the sustainable development of an economic entity.