The deformation of initial solid shell has important effects on the formation of surface defects of continuously cast slabs of steel. In order to understand the thermal deformation behavior of initial solid shell of steel, droplets of Fe-C alloys and commercial steels were solidified on a chill plate and the bottom shape of solid shell was measured. The solid shell always deforms during solidification and cooling to give a spherical bottom shape. This deformation of solid shell shows a very strong dependence on carbon content. When carbon content is lower than about 0.6 mass% or higher than 2.5 mass%, solid shells show positive deformation, i.e. convex towards the chill. When crbon content lays between 0.6-2.5 mass%, solid shells show negative deformation, i.e. concave towards the chill. Two peaks of positive deformation were found as carbon contents come near to 0 mass% (pure iron) or 0.12 mass%. These two peaks of positive deformation of solid shell correspond to the reported peaks of frequencies of surface defect occurrence such as longitudinal cracks and deep oscillation mark of continuously cast steel slabs. The deformation of pure iron solid shell was accurately pridicted with a deformation model of initial solid shell previously derived by the authors. An explanation for the maximum deformation at 0.12 mass% carbon conent was also made by the model using a numerical heat transfer calculation with a coupling of deformation and solid shell-chill interface heat transfer coefficient.