Solving a complex problem often requires a way to break it down into smaller, interconnected and manageable sub-problems, and then to join them together. The concept of breaking down a problem into smaller pieces is generally referred to as decomposition. The design of mechatronic systems is an example of such complex problems, as it is based on the integration of several disciplines, such as mechanical, electrical and software engineering. Decomposition is thus a common technique to help designers to obtain solutions for the design of mechatronic systems during the systems engineering process. However, an effective decomposition method which can fully solve the design problems of mechatronic systems has not yet been proposed in systems engineering. The goal of the paper is to formalise this decomposition method based on an interface model. This method is applicable to the architecture definition in the context of the design of mechatronic systems during their conceptual design phase. The proposed decomposition method provides designers with high-level guidance to help them to achieve the appropriate hierarchies and granularities for the architecture of mechatronic systems. The proposed decomposition method is applied and demonstrated using the systems engineering practices of a 3D measurement system.