Vehicle thermal management is a key issue for improving the fuel consumption. Though simulating the thermal behavior and predicting the contribution on fuel economy are significantly effective, the simulation requires large amount of data that is not available in the early phase of vehicle development. This study presents a practical methodology for powertrain thermal modeling. Adding the model to a vehicle dynamic simulation made it possible to visualize the detailed energy flow within the vehicle and to predict the fuel consumption during driving cycles including cold-start condition. The simulation approach is characterized by introduction of optimization techniques for identifying parameters. The model was validated with measured warm-up behavior. The model provided satisfactory accuracy and demonstrated the ability of this approach.