This thesis project is the result of evaluating the possibilities to use computer fluid flow simulation programs to develop cooling system models for a new generation of Volvo cars. The work was commissioned by Volvo Car Corporation (VCC) in Gothenburg, Sweden. Today's shorter and shorter development time and demands of lower costs of cars sets new goals for the technical tools in use of design. It is therefore of interest to investigate new computer tools for cooling system design. Such a solution could in the future give shorter development time and reduce expensive physical testing. In this thesis project the software Flowmaster was analysed and used to design the cooling system model of a RN3T engine and its components. The software is a 1-D network fluid flow solver that allows engineers to analyse a comprehensive range of engineering problems. Fluid networks of almost any size or complexity can be analysed rapidly and accurately in order to establish design integrity. The software is a single integrated system capable of solving steady state, transient, compressible flow, fluid power and heat transfer problems. The program is developed by Flowmaster International Ltd. To collect knowledge about the cooling system and its design an analysis was made of the engine and its components. Then a computer fluid flow model was created for different driving conditions, to simulate the flow and pressure. To verify the computer model a physical test was done on a RN3T engine. The physical testing was carried out with respect to the coolant flow, pressure and temperature. The simulations showed that the software Flowmaster is well suited to fit the commands of cooling system simulations. Using Flowmaster engineers are able to concentrate on the engineering problem to produce the best designs. Whether they are designing automotive cooling systems, fuel feed systems for engines or some other application. Volvo Cooling System department has decided to introduce Flowmaster in their design work to gain valuable time.