We examined the effect of melt overheating on primary solidification in aluminum alloys of various systems. The alloys were poured from the temperature of overhearing, and no special treatment of the melt was performed. We assessed the sizes of equiaxed and columnar grains or primary crystals and the size of dendritic cells. The experimental results show that the size of main primary crystals depends nonlinearly on the overheating temperature, One can observe the growth of primary crystals and their following refinement, the latter occurring at considerably high pouring temperatures depending on the alloy composition and the cooling rate. The observed results were interpreted in terms of the formation of solidification nuclei upon decomposition of a supersaturated during undercooling liquid solution (melt).