Function is the most crucial concept in designing better and more innovative products. Function modelling is a critical step in the product design process. A modelling technique using the Petri Nets (PN) for function representation is presented based on the black box method and the theory of function basis. The characteristics of function modelling are summarized firstly. Then the feasibility of function modelling based on PN is demonstrated. The PN model provides easy graphical representation for visualization of function analysis. Finally the example of attaching a cap to a tube of toothpaste using automated equipment is given to illustrate the modelling technique of conceptual design. The functional model established in this work can completely describe the dependencies between flows and functions, and will make computer tools available that support functional design and innovative design in the future.