Effect of Mg and Ti addition on the solidification macrostructure of high purity ferritic stainless steel was evaluated in order to obtain the fine grains. In this work, columnar to equiaxed transition could not be obtained by the addition of Mg and Ti. This is due to the small constitutional undercooling in this alloy system. However, fine columnar grains were obtained by adding Mg and Ti. It is thought that the larger number of inclusion particles for heterogeneous nucleation is related to obtain the finer grain structure at the initial stage of solidification with large undercooling. Moreover, grain selection behavior of columnar grains has been also analyzed in this work.