This paper presents an Organic Rankine Cycle (ORC) system power optimization for heavy-duty diesel applications. The optimization process is carried on an experimentally validated dynamic, physics-based model, which mainly consists of parallel-connected tail pipe and EGR evaporators, a high pressure working fluid pump, a turbine expander, etc. Different vapor temperature reference trajectories are set to optimize the ORC system net power, which is the difference between turbine generated power and the power consumption by high pressure pump and condenser coolant pump. A constant speed variable load (CSVL) cycle is considered as engine operating conditions during the optimization process. Optimization results reveal that in certain range lower constant vapor temperature reference trajectory shows higher net power generation. However, if the vapor temperature trajectory is within 40 oC range to the working fluid saturation temperature, the net power starts to decrease. Constant superheating temperature reference trajectory shows the same trend as constant vapor temperature reference strategy while the maximum net power is 12% higher, which is the results of adaptive reference trajectory to the evaporation pressure. This results will be validated in the experiments later and act as the benchmark for the ORC system net power optimal control, such as model predictive control.