Directional solidification experiment has been carried out on Fe-3.4wt.%C-2.0wt.%Si alloys with and without the addition of 0.02wt.%S. The effect of O on the transitions in graphite morphology was investigated by controlling the O₂ partial pressure of solidification atmosphere. The use of pure Ar, or 10 volume %O₂ or H₂ plus balance Ar atmosphere has produced a definite effect on the graphite morphology solidified from high purity alloys. Contrary to the results of other works oxygen was found to promote coral growth by suppressing flake graphite, while hydrogen was counteracting the O effect. The addition of S to the high purity alloys greatly promoted the formation of flake graphite regardless of the solidification atmosphere, indicating that the effect of O on graphite morphology was neutralized in the presence of S.