To increase the casting quality of hypoeutectic Al-Si alloys, the effects of melt thermal treatment on the solidification structure of the A356 alloy were analyzed by a factorial experiment, in which the overheated melt was mixed with the low temperature melt. Experimental results show that the elongation ratio and strength of the treated samples increase remarkably compared with the control sample. The primary dendrite size reduces dramatically and the dendrite changes from columnar to equiaxed, with a little change of the secondary dendrite arm spacing (SDAS).Combined with the measurement of the nucleation undercooling, it is concluded that the solidification structure and refining effect are dependent primarily on the low temperature melt. The refining mechanism is believed as a result of the multiplication of the nuclei in the melt thermal treatment procedure.