Abstract The Hybrid Electric Vehicle (HEV) powertrain comprising Electric Motors, High Voltage Power Electronics, High Voltage Battery Pack and an on-board generator (Auxiliary Power Unit – APU) in case of Range Extended – Electric Vehicles (RE-EV) require appropriate cooling. Based on the cooling requirements, different cooling system architectures, heat exchanger arrangements, battery cooling and cabin cooling / heating methods have been evaluated using a holistic system modelling approach with 1D simulation software Flowmaster™(1). This paper will present the approach and process for evaluating different cooling architectures, the simulation methodology, and how the simulation can help to understand the thermal issues encountered on an HEV and related architectures.