The project target is to design and validate the cooling system model of a CNG engine for Off Road applications, and to perform fluid dynamic and thermal simulations for predicting the system performances at different engine operating conditions. The model, created using a unidimensional approach, is designed in order to be used either in the prototyping phase, to test new technical solutions and design proposals, or to predict the cooling system behavior within the customer vehicle. An experimental activity at the engine test bench has been carried out to provide the target data for the model calibration and to compare the simulations results with the target measured quantities.