The methodology of calculating parameters of primary and secondary non-metallic inclusion formation in thermite alloys is offered. The regularities of the growth time of non-metallic inclusion in the form of corundum depending on its size, mass and quantity are analysed. It is shown that in the thermite alloy obtained by self-propagating high-temperature synthesis, the average size of corundum inclusions, surrounding the heterogeneous inclusions of chromium carbide, is 15.4 μm. It is shown that during the process of aluminothermic reactions of SHS process a significant number of small inclusions of corundum is formed. It is shown that the alumina particles are dissolved in the alloy and they are the centers of crystallization and play the role of inoculating modifier.