Al–5.5%Mg–2.3%Si–0.6%Mn alloy and Al–13%Mg₂Si pseudo-binary alloy were cooled at various cooling rates during solidification. Morphological change of secondary particle was examined. Solidified structure of Al–5.5%Mg–2.3%Si–0.6%Mn alloy consisted of primary α–Al dendrites, Al–Mg₂Si eutectic structure and Al₆Mn particles. The most significant morphological change was from lamella Al–Mg₂Si eutectic structure in the permanent mold cast and the die-cast product to refined and globular Mg₂Si phase in the high-speed twin-roll cast product. In contrast, no morphological change was observed for Al₆Mn particles. In order to investigate the effect of morphological change of Mg₂Si phase on tear toughness, tear toughness tests were performed for Al–13%Mg₂Si pseudo-binary alloy cast products produced by permanent mold casting and high-speed twin-roll casting. Unit crack propagation energy (UE_p) of the high-speed twin-roll cast products was 5 times higher than that of permanent mold cast products. For permanent mold cast specimens, crack propagation occurred along the interface of plate-like Mg₂Si/matrix of the eutectic solidified region and in Al-matrix. But fracture surface of high-speed twin-roll cast specimens was covered with fine dimples. Such difference of fracture mode is due to morphological change of Mg₂Si phase from plate-like to globular by rapid cooling.