As a result of the analysis of the formation processes of fluidity and the conditions for feeding the castings during cooling after solidification, a significant effect of the properties of the melt and the dispersion of the primary structure on the fluidity and density of steel was established. It is theoretically substantiated and experimentally confirmed that the level of fluidity and density is determined by the dispersion of the dendritic structure, the magnitude of the melt overheating over the liquidus temperature, as well as the properties of the liquid metal, the thermal conductivity of steel at the solidus temperature, crystallization heat and crystallization interval. The established quantitative laws describe the real process with a probability more than 95% and a high degree of reliability (R = 0,709-0,837; ð = 1,2 – 13,8%). The article shows that in order to increase the effectiveness of the influence of integral factors on the fluidity of structural steels, they can be arranged in the following sequence: thermophysical conditions of solidification, dispersion of the dendritic structure, properties of liquid metal. In this case, an increase in fluidity occurs with an increase in the overheating of the melt above the liquidus temperature, the heat of crystallization and the dispersion of the dendritic structure. An increase in the values of other factors leads to the opposite effect. Alloying elements are arranged in the following sequence: Si, Cr, Mn, C, V, N, V + N according to the specific efficiency of increasing fluidity. The results of the studies performed show that according to the effectiveness of the influence of the considered factors on the steel density, they can be arranged in the following sequence: dispersion of the dendritic structure, properties of liquid metal and thermalphysic conditions of solidification. Alloying elements affect these parameters in such a way that a complex multiextremal change in density is observed during alloying of steel. The general trend is that carbon and chromium decrease, while silicon, manganese, vanadium, nitrogen, and co-alloying with nitrogen and vanadium increase the density of the steel. Alloying elements can be arranged in the following sequence: V, Cr, Mn, Si, N, N + V, C to increase the specific efficiency of changing the density.