Fuel cell represents a new area of interest for power system engineers and researchers. This paper presents control-oriented physics-based modelling of fuel processing system (FPS) reactors in fuel cell power systems. Basically, the FPS converts hydrocarbons to hydrogen rich gas and cleans harmful species according to fuel cell requirements. The physics-based model has importance of understanding the system and useful for model based control designs. The model also can be used for model-based observer designs to estimate unmeasured states or eliminate the sensors.