<p>Computer simulation methods were used for modeling the process of dissolving powder briquettes-modifiers of various sizes and configurations during in-mold modification of cast iron. The modeling included a combined thermophysical and hydrodynamic calculation. The heat transfer conditions on the surface were set by boundary conditions, and the heat transfer in the volume was described by the non-stationary heat conduction equation. The modeling of hydrodynamics (melt flow) was carried out based on the Navier–Stokes flow equation. The finite element method was used to solve the system of these differential equations. The modeling results using the developed model showed that the optimal shape of the briquettes-modifiers is a sleeve with an internal conical hole. The use of such a briquette shape for modification due to the increased level of hydraulic resistance for the metal melt at the initial stage of pouring provides an increase in the pouring time and contributes to the rise in the degree of magnesium absorption and a more uniform distribution of the modifier in the casting volume. The reliability of the modeling results is confirmed by the results of the resulting experimental data.</p>