Internal cracking in DC casting billet of high strength Al-Mg-Si alloy was investigated. The internal crack occurs in the mush zone, and the fractured surfaces of DC billet have a rupture structure where intergranular fractures with remaining liquid around interdendritic regions were observed. Furthermore, an unsteady-state thermal conduction analysis in the casting process of billet was performed. The effects of solidification parameters such as the cooling rate and thermal gradient on internal crack in DC billet were analytically investigated. By comparing the calculated results with the experimental ones, the criterion of internal crack was quantitatively determined. Main conclusions obtained are; (1) Starting point of crack is considered to be at the position about from the starting level in the center of the billet. (2) The difference of cooling rates between the surface and center was one of the criteria for crack generation. When the difference exceeded 6.0K/s, a crack is generated. It is considered that the difference of thermal gradients between the surface and center is possibly one of the criteria for the crack formation. When the difference exceeded 3500K/m, the billet will be cracked.