Abstract: Thermal conditions forming channel segregation in a steel ingot depend not only on cooling rate but also on the ratio of axial temperature gradient, GA? to radial one, GR. If the ratio GA/GR increases, which is favourable to axial directional solidification, the segregation is well reduced. The reduction of the segregation is explained by the change of the height of mushy zone, H, under different conditions. Criterion GrD, modified by D'arcy law, is used to show the characteristic of natural convection in the mushy zone. The smaller the height H, the smaller the criterion GrD and the weaker the interdendritic, flow causing the channel segregation. Experimental results show that to make the steel ingot solidified directionally from bottom to top is one of the effective methods preventing the channel segregation.