In order to exhibit good all-round performance the impact toughness enhancement of cast high-speed steels (HSS) is obligatorily needed. In general, different methods are used commercially to achieve cast structure refinement and, as a consequence, their properties are improved. Introduction into the melt of inoculants particles or surface-active additions is among most beneficial. However, the effect of modifying additions in cast HSS has been studied insufficiently. For this reason the theoretical evaluation and experimental confirmation of the modifying and refining effects of 23 chemical elements in HSS have been carried out. The relationships between the structural parameters and mechanical properties in the M2 and T30 HSS have been established.