Hybrid electric vehicle (HEV) is regarded as one of the most promising vehicles for the next generation vehicles, while the control strategy is highly involved to improve the collaborations of the different components due to its complexity and a significant role playing in digging out all the potentials of fuel economy for HEVs. This paper presents a novel methodology to generate optimal control law for an HEV equipped with a diesel engine. The establishing of the simplified model is introduced firstly, followed by the selection of the state variables for the system. Afterwards, an objective function is defined over a specific drive cycle to minimize the fuel consumption, as well as to take the engine out emissions into consideration and to avoid the frequency shifting. Dynamic Programming (DP) technique is employed to search the optimal control law with objecting to several physical constrains. The simulation results show the effectiveness and reliablity of this method. Finally, the results of different pair of weighing factors are compared to make a tradeoff between fuel consumption and emissions. And the contrast indicates that emissions could be decreased dramatically at the expense of a small increase in fuel consumption.