The directional solidification and S/L interface stability condition were studied in the horizontal continuous casting of 8mm diameter aluminum rod adopting the heating mold. The directional solidification could be observed at the growth rate R of 10 to 240(㎜/min) and G₁/R of 36.6 to 2.1(℃· sec/㎟). The S/L interface transition from planar to cellular, and, finally, to fine cellular interface could be observed as G₁/R decreased. However, no interface breakdown to dendrite or equiaxed structure could be observed. The striations parallel to the growth direction occurred throughout the given range of growth rate by increasing in amount as the growth rate increased. These striations were considered to be a kind of twin introduced during growth from melt. As the amount of striations increased, the tendency for recrystallization on annealing increased. This fact indicates that these striations are acting as the source of nucleation sites.