In this paper, an energy management system of power-split Plug-in Hybrid Electric Vehicle (PHEV) is designed. This PHEV with power-split drivetrain operates in discrete operating domains. The drivetrain dynamics of the system are continuous within each of these discrete modes. This PHEV system consisting of discrete operational modes and continuous dynamics can be modeled using the hybrid dynamical system framework. In this paper, the vehicle and powertrain dynamics of power-split PHEV are introduced. Using these dynamics, a hybrid system model of the PHEV is proposed, and a nonlinear constrained energy minimization problem is solved using the dynamic programming approach. This designed Energy management system's performance is evaluated for different city and highway driving.