Short-term optimal hydrothermal scheduling problem is one of the most popular research issues in power systems optimization. A novel mathematical model of the short-term optimal hydrothermal scheduling is proposed in this paper. This model aims at minimizing the total fuel cost of the thermal generating units while satisfying the various constraints such as power balance, water balance, transmission network and other system's constraints. A modified differential evolution algorithm is also introduced to solve the short-term optimal hydrothermal scheduling problem. In the proposed approach, an operation of migration and a self-adaptive mechanism are presented to improve the searching efficiency. Moreover, four constraint handling rules are proposed to handle the complex constraints of short-term optimal hydrothermal scheduling problem. An IEEE nine buses test system is applied to verify the proposed mathematic model and algorithm. The numerical results show the feasibility and efficiency of the proposed approach to the short-term optimal hydrothermal scheduling problem.