Stripping and mining operations at open-pit coal mines are performed mainly by heavy shovel-truck systems (STS). One of the main problems of the STS is a rather low level of its operation quality, an objective assessment of which is an important step in identifying the causes of low quality and effective ways to improve it. The purpose of assessing the STS operation quality is defined as a functional criterion. The next important step of the assessment is to choose the set of indicators that most characterize the STS operation quality. In this article we present the rationale, the general principles for the formation of quality indicators set, the sources and the main dependencies for its determination. For this purpose, modern methods of data collection and processing, analysis and synthesis are used. The ability to assess the STS operation quality is very important for identifying the main directions of improving its operational performance, reaching the optimization of the key performance indicators by the quality criterion, and, as a result, for the possible saving of material and technical resources in the open-pit mining of minerals.