Purpose. To develop a new approach to evaluating the complexity of the structure of technological systems of mining and metallurgical complex with the proposed integrated index. The practical application of this indicator is due to the methodological difficulties arising when determining the hierarchical level of the elements of the systems studied. Methodology. A systematic approach is applied that allow us to explore industrial complexes of equipment as systems objects and present them in the form of technological systems. Methods of analysis and synthesis are used that allow us to identify common elements in the known methods for estimating the complexity of systems and to develop a new methodological approach to the process of decomposition systems. Methods of simulation of technological systems are applied, which allow submitting them in the form of models structural-element schemes. Findings. A new methodological approach is proposed to quantify the complexity of the structure of technological systems of mining and metallurgical complex, in which the process of decomposition of the system is performed by the method of successive cut-off of element connections with the system. Using the proposed decomposition method provides high accuracy and reliability when comparing technological systems with the structure of different hierarchical levels and consisting of a different number of subsystems and elements. Approbation of the developed method on the example of the pelletising plant pellets No. 2 of Pivnichnyi Mining and Processing Plant (Kryvyi Rih). It is shown that in the real technological systems the complexity of the structure occurs mainly due to the series connection of extra equipment rather than creating new relations. To increase the index of relative complexity of the technological system management structure, it is reasonable to use a combined series-parallel connection of additional technological equipment, which ensures the highest hierarchical level of the elements in the system. Originality. The originality of the new approach to assessing the complexity of structure of the technological systems of mining and metallurgical complex is to develop a method of decomposition of the system, the essence of which consists in the successive cutting off of the connection of the elements with the system. Practical value. The practical value of methodological approach the developed by the authors to estimate the complexity of the structure of the technological systems of the mining and metallurgical complex is that in contrast to the known method, the quantitative evaluation of system structure complexity is performed without performing the operation of the formal description of the structure. It provides high accuracy and reliability of the result, reduces the complexity of the evaluation process.