Solid oxide fuel cells (SOFC) operate under high temperature conditions compared with the other types of fuel cells. High efficiency and fuel impurity tolerance are the major advantages. However, this kind of high temperature power generators always suffers from slow start problem from atmospheric condition. A typical 5 kW SOFC takes about one hour to start, provided that all the operation parameters are tuned very well. This kind of transient performance is still very difficult to extend the applications to automobiles. To reduce the cold start duration, a low temperature SOFC electrolyte is the major development to overcome this problem. However, from system dynamics viewpoint, an observer can be developed to monitor the transient performance, control parameters, such as fuel inlet flow rate, can be tuned accordingly. Since temperature is the major factor that influences the SOFC transient, a sliding observer was designed in this paper to trace the stack temperature as well the energy flow. A hardware-in-the-loop (HIL) test rig was set up, and the simulation results show that the cold-start lag can be further reduced by proper control strategies.