The initial stage of solidification for continuous casting is very important in optimising the surface quality of steel. It is well known that hypo-peritectic steel has problems with longitudinal cracking due to shrinkage resulting from the δ→γ transformation. High speed continuous casting, with a speed range of 4-5 m/min, suffers from the same problem. For a round-shaped billet caster, the shape of the mold cross section is at a disadvantage with regard to longitudinal cracking. In this study, mild cooling mold powders were developed in order to improve the surface quality of high speed casting slabs and round-billets. The results can be summarized as follows; 1) High CaO/SiO2 ratio mold powder with a high solidification temperature is most suitable for a slab caster. 2) Low CaO/SiO2 ratio mold powder with a high solidification temperature is most suitable for a round billet caster. 3) The effect of radiation in liquid powder film is bigger than solid powder film.