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In order to demonstrate the transformation relation of two primary Al 13 Mn 4 Si 8 and Al 15 Mn 3 Si 2 phases during solidification of Al-12Si-4Cu-2Mn (in wt%) alloy, verification experiments through controlling cooling rate of solidification and quenching at different stages during solidification were designed and the microstructure was characterized by optical microscope, scanning electron microscope and transmission electron microscope. The cooling rate during solidification plays a decisive role in the formation of primary Mn-rich phases. Fast cooling rate favors the slender rod-like Al 13 Mn 4 Si 8 phase and slow cooling rate leads to the formation of dendritic Al 15 Mn 3 Si 2 phase. A great number of symbiotic complexes, constituted with inner Al 13 Mn 4 Si 8 and outer Al 15 Mn 3 Si 2 phases, are found in the transition area of the surface and center of the casting with a middle cooling rate. Microstructure observation of the liquid quenching samples demonstrates that the first phase formed during solidification must be 2D lath-like Al 13 Mn 4 Si 8 phase whether with high cooling rate or with slow cooling rate. And 3D dendritic Al 15 Mn 3 Si 2 phase is transformed from 2D Al 13 Mn 4 Si 8 by a peritectic reaction: L+ Al 13 Mn 4 Si 8 →Al l5 Mn 3 Si 2 , which is dependent of the cooling rate during solidification. A schematic model with double nose curves was established to exhibit the influence of cooling rate on the peritectic reaction. With sufficient high cooling rate, this peritectic reaction will be suppressed completely. When the cooling rate is slow enough, the peritectic reaction will proceed completely. However, in case of middle rate, the peritectic reaction will happen but incompletely. [ABSTRACT FROM AUTHOR]