The dynamic performance of a reformer has a major impact on the safe and efficient operation of an overall molten carbonate fuel cell power-generation system. Hence, an investigation on the dynamic characteristics of a reformer with regard to its operation in the fuel cell systems is carried out. A numerical model is developed and it is capable of calculating the dynamic responses with regard to load-following modes and disturbances. In particular, the model can consider simultaneously the dominant processes of a reformer, such as chemical reactions and heat transfer as well as mass-transport. In this paper, the dynamic responses of the processed gas from the reformer corresponding to the step changes are performed, and these responses have provided valuable insight into the characteristics of the reformer operating in fuel cell systems.