We present a density functional theory approach to investigate the thermodynamics of ice nucleation in supercooled water. Within the theoretical framework, the free-energy functional is constructed by the direct correlation function of oxygenoxygen of the equilibrium water, and the function is derived from the reference interaction site model in consideration of the interactions of hydrogenhydrogen, hydrogenoxygen, and oxygenoxygen. The equilibrium properties, including vaporliquid and liquidsolid phase equilibria, local structure of hexagonal ice crystal, and interfacial structure and tension of waterice are calculated in advance to examine the basis for the theory. The predicted phase equilibria and the waterice surface tension are in good agreement with the experimental data. In particular, the critical nucleus radius and free-energy barrier during ice nucleation are predicted. The critical radius is similar to the simulation value, suggesting that the current theoretical approach is suitable in describing the thermodynamic properties of ice crystallization.