Transient thermal change in directional solidification (DS) castings with cylindrical geometries was numerically calculated by finite difference method considering radiation heat transfer. The temperature distribution along the DS ingot center line and outer mold surfaces was experimentally measured to be compared with the calculated values, and the validity of the analysis was confirmed. Together with macrostructural observations at vertical cross sections of the DS ingots, the analysis suggests that liquidus or solidus lines must be maintained horizontal or convex during the entire DS process in order to suppress new grain nucleation at the cooled mold wall, and thus to promote continuous growth of the columnar grains from the chill surfaces.