The influence of the carbon content on secondary dendrite arm spacing has been examined. From the cooling curves for various locations in the end chilled ingot, the local average cooling rate during solidification is measured. It is found that over the range of the carbon contents studied, 0.14∼0.88%C, secondary dendrite arm spacing, S μ, in cast steels is independent of the carbon content but influenced only by the average cooling rate, R°C/min. The relationship for the carbon steel is S=710R^−0.39. It is shown that this relationship is also valid in low alloy steel containing 1.17%Cr and 0.25%Mo.