The cooling system of Series Hybrid Electric Vehicles (SHEVs) is more complicated than that of conventional vehicles due to additional components and various cooling requirements of different components. In this study, a numerical model of the cooling system for a SHEV is developed to investigate the thermal responses and power consumptions of the cooling system. The model is created for a virtual heavy duty tracked SHEV. The powertrain system of the vehicle is also modeled with Vehicle-Engine SIMulation (VESIM) previously developed by the Automotive Research Center at the University of Michigan. VESIM is used for the simulation of powertrain system behaviors under three severe driving conditions and during a realistic driving cycle. The output data from VESIM are fed into the cooling system simulation to provide the operating conditions of powertrain components. The cooling system model includes various component models for three main fluid circuits of coolant, cooling air, and engine oil. The model predicts the thermal responses of all cooling system components and the temperatures of the engine and electric components. Using the cooling system models, the thermal response and power consumption of the cooling system over a realistic driving cycle is estimated and the factors that affect the performance and the power consumption of the cooling system are identified.