The air gap between the metal and mold, formed by shrinkage during solidification, causes surface and subsurface cracks in the continuous casting process. Molten crack on the surface might also occur due to improper heat transfer between them. In order to compensate the air gap in mold design, the thermal contraction is an essential factor. In this study, the thermal contraction and expansion behaviors were examined from the ( α and pearlite)/ γ to γ / δ transformations in continuous casting steels by the commercial dilatometer and the self- assembled dilatometer with laser distance measurement. It was found that the thermal contraction and expansion behaviors were very dependant on the phase transformation of the γ / δ as well as ( α and pearlite)/ γ . The sudden volume change from δ to γ which might cause cracks in the continuous casting process, was observed on cooling just below the melting temperature by the self-assembled dilatometer.