Optical metallography and electron microscopy were used to comparatively examine the structures of high-strength silumin produced by the conventional technology of smelting the charge in an electric furnace in air and by a new method of synthesis in a melt of halides. The classification of structural macrodefects observed in the castings produced by the conventional method is proposed; the dependence of the mechanical properties of castings on the defect concentration has been established. Advantages in favor of the new technology of synthesis of high-strength silumins, which ensures a modified structure and high mechanical properties of the material, are given.