Abstract: For the high temperature thermodynamic properties, the differences between the casting steel and the rolled steel are very significant, and the differences between the different steel grades are also very obvious. In this research, based on the microscopic theory of solute segregation, and with the combination of the experimental test and calculation analyses, the high temperature thermodynamic properties of SS400 steel and 65Mn steel for casting condition had been researched. It is shown that, the equivalent thermal conductivity, the equivalent specific heat and the Poisson’s ratio are decided by the width of the corresponding solidification two-phase zone; the Young’s modulus of the casting steel could be fitted by the high temperature tensile test results; the coefficient of thermal expansion is decided by the liquid impenetrable temperature and the metal tissue types, for 65Mn steel, when solidifying, it forms γ-austenite directly, so its maximum coefficient of thermal expansion is bigger than SS400 steel, and reached 0.00036/℃.