A solution entropy model was developed for the undercooled binary eutectic alloy systems. As an ex-tension of Taylor and Fidler et al.’s model, the present model considered the change of phase compo-sition with the increase of undercooling. Furthermore, the sub-regular solution model and the interac-tion parameter (IAB) were also introduced. In this paper, the extended model is used to calculate the solution entropy for binary eutectic phases under the undercooled condition, and the application scope of the model is also extended. Not only the growth manner of eutectic phases, but also the transition of morphologies may be predicted and explained by calculating the solution entropy of eutectic phases under the non-equilibrium condition with the developed model. Experimental results show that the developed model is valid for the undercooled Ni-Si and Ni-Sn eutectic alloy systems. As an ex-tension of Taylor and Fidler et al.’s model, the present model considered the change of phase compo-sition with the increase of undercooling. sub-regular solution model and the interac-tion parameter (IAB) were also introduced. In this paper, the extended model is used to calculate the solution entropy for binary eutectic phases under the undercooled condition, and the application scope of the model is also Not only the growth manner of eutectic phases, but also the transition of morphologies may be predicted and explained by calculating the solution entropy of eutectic phases under the non-equilibrium condition with the developed model. Experimental results show that the developed model is valid for the undercooled Ni-Si and Ni-Sn eutectic alloy systems.