The design of a vehicle's cooling system during its development phase requires a considerable amount of time and effort, especially when matching up the individual components. Due to the increasing complexity of cooling systems of diesel engines, it becomes more difficult to develop and optimise their performance. In determining the constructional details and optimising them, it is necessary to consider the interaction of the individual components as well as the complete system. The necessary experimental work required in the course of the system development is thus vastly increased. In order to support testing activities and do minimize the time spent on development, a simulation program was prepared which makes simulating the operating conditions of the most different cooling system possible. In this study, performance of a diesel engine cooling system is analysed including the influences of the cooling system components and parameters.