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The phenomenon that the structural characteristics in raw materials reappear in castings during solidification is called the structural heredity of materials. But so for, the mechanism of structural heredity has not been clear yet. In order to study the mechanism of structural heredity, it is necessary to discuss the structure of liquid metal or alloy. It is believed that there are so-called pseudo-nuclei or embryos in liquid metal or alloy. This is the precondition of nucleation in the process of liquid-solid transform. The arrange of atoms is irregular in long range, but regular in a microregion in the twinkling of an eye. The regular arrange of atoms in liquid metal or alloy establishes the theoretical basis of structural heredity. The embryos, at this circumstance could be definited as dispersive particles which are potential grain nuclei and have hereditary relation to the original solid crystal. The number, composition and distribution of dispersive particles mainly depend on the structural characteristic of original crystal. The number of dispersive particles per unit volume is in direct proportion to the number of grains per unit volume under certain condition. The highest temperature at which dispersive particles can still exist is definited as Tm. If liquid temperature is below it, the dispersive particles are kept and most of them may become grain nuclei, the structural heredity exhibits. Otherwise, the dispersive particles are dissolved. the structural heredity will disappear. This paper mainly deals with the mechanism of structural heredity in ZL104 alloy, a commercially important hypoeutectic Al-Si alloy (ZL104), having the composition of Al-9.40 % Si-0.35 % Mg-0.36 % Mn-0.42 % Fe. In this alloy, there are an alpha phase and an eutectic silicon phase. The latter has much more hereditary effectiveness than alpha phase at a certain temperature this is because the Tm of eutectic silicon phase is higher than that of alpha phase. The dispersive particles of silicon phase play therefore an important role in structural heredity. There is an upper limit of temperature for dispersive particles which is definited as Tm. If melting temperature is over Tm, the structural heredity will disappear.