The development of mechatronic products is complex because it deals with the interaction between different domains. In a literature review, the absence of a method capable of integrating the functional view of a mechatronic product into a single model was observed. This article aims to develop a functional modeling method to aid the conceptual design of mechatronic products. The proposed procedure describes the phases and steps required to create functional models and use cases for the different mechatronics knowledge domains and to combine them into a single functional model. The use of the proposed method is demonstrated through its application in the development of an air compressor. The method is then evaluated by experts and tested in the development of an autonomous vehicle for railway inspection, demonstrating its feasibility and the presentation of functional information in a single diagram. Factors such as effectiveness, usability and repeatability were also verified, and an efficient presentation of the function model was observed, allowing to view the relations between the mechanics, electronics and software domains in a unified diagram. The great benefit of the proposed method lies is in the possibility of also using functional models in the context of mechatronic product design.