This paper deals with a new bondgraphs representation method for pneumatic systems. A pneumatic system has compressible fluid-flow and thermal fields. In constructing bondgraphs model for such a system, fluid and thermal power bonds have been employed. In addition to it, multi-port C and multi-port R elements have been frequently used. Therefore, the resulting bondgraphs model becomes complicated, and it is difficult to understand how energy flows branch off or join together. From this viewpoint, a bondgraphs representation for such a system is incorporated into the present study by introducing a new concept of effort and flow in both compressible fluid-flow and thermal fields. In this bondgraphs representation, the product of effort and flow is power, which means that true bondgraphs are being used. Furthermore, only 1-port C and 1-port R elements are used for modeling of this system. Therefore, the energy flows can be easily understood from the resulting bondgraphs representation. To confirm the usefulness of this representation for a simple pneumatic system, simulation was carried out. By comparing the simulated results obtained with the experimental ones, the usefulness of this representation is clarified. In simulation, a simulation program named BGSP was employed without modifying it so as to use multi-port elements. It is a great achievement that the modification of BGSP is not needed when this representation is used in modeling a pneumatic system.