This paper presents an optimal control strategy of a parallel Plug-in-Hybrid Electric Vehicle which minimizes the vehicle’s fuel consumption including optimal engine on/off decisions. For that purpose a consistent approach based on Pontryagin’s Minimum Principle is applied by considering future driving cycle information. The paper discusses an iterative method which involves two steps: First, an analytical co-state for a given engine on/off strategy is calculated. Second, an update of the engine on/off decision points based on the calculated co-state is performed by comparing the associated Hamiltonians for the corresponding engine on/off modes. Through the compact analytical nature, the developed approach is computationally efficient and facilitates the real-time implementation on control units employed in series-production parallel Plug-in-Hybrid Electric Vehicles.