The microstructure and solidification process of three Al-Mg-Si alloys with different magnesium contents have been studied using optical microscopy and the electron probe X-ray microanalysis. The results showed that Al-Mg-Si alloys possessed fairly complicated solidification path : L-->alpha-Al + L(1)-->alpha-Al + Al15Si2(FeMn)(3) + L(2)-->alpha-Al + Al15Si2(FeMn)(3) + (alpha-Al + Mg2Si) + L(3)-->alpha-Al + Al15Si2(FeMn)(3) + (alpha-Al + Mg2Si) + (alpha-Al + Mg2Si + Al15Si2(FeMn)(3)), and wide solidification temperature of 75 degrees C. The magnesium content in the alloys greatly influenced the as-cast microstructure. The higher the magnesium content, the more Mg2Si structure was present. Iron and manganese segregated to the finally solidified zone, which resulted in the formation of ternary eutectic structure. Although their content in the alloys was very low, their effect on solidification behaviour cannot be ignored.