The master thesis consists of a modification of an existing cooling system through the die casting tool outer die. Solutions for cooling system are searched in various fields of engineering. The final version of a new tool design needs to be compared with an existing one. For that reason, a heat transfer and thermomechanical analysis were needed for comparison between existing and new tool. For a conceptual choice that will guarantee a good sealing solution on a transition of cooling channels between outer and inner die, an O-ring was chosen as it presents the best solution based on the rating scale. Temperature measurements were performed in order to validate the results of heat transfer analysis. Based on that, we could confirm that the results from the analysis were a good approximation to real ones. The heat transfer showed the main differences between the existing and new tool. It showed how cooling channels affect the distribution of temperature over the tool. Due to this difference, it also affects the results of a thermomechanical analysis. The main differences are shown in the distribution of stresses and strains.