A major element of modern development methods for mechatronic systems is a consistent model-based design. Models are used in various fields of this process. The difficulty lies in the modelling that can be very intricate in parts. Structure and type of modelling are not formally described and therefore depend on the developer of the model. For a model to meet the required modelling objectives efficiently, it is essential to select an appropriate modelling depth. This thesis presents a novel method for the systematic selection of a modelling depth in the design process of mechatronic systems. The method makes an important contribution to improving the design process of mechatronic systems and supports the developer in choosing an appropriate modelling depth at the key design stages. For identifying these design stages, a methodology for the design process of mechatronic systems is elaborated. According to the design methodology, the method for selecting the modelling depth is integrated to the design process where required. Both the design methodology for mechatronic systems and the method for selecting the modelling depth are applied using the example of an ATM.