This paper studies a control algorithm for fuel cell/battery city buses. The output power of the fuel cell is controlled by a D.C. converter, and the output ports of the converter and the battery are connected in parallel to supply power for the electric motor. One way to prolong service life is to have the fuel cell system to deliver a steady-state power. However, because of fluctuations in the bus voltage and uncertainness in the D.C. converter, the output power of the fuel cell system changes drastically. A closed-loop control algorithm is necessary to eliminate the errors between the output and target power of the fuel cell system. The algorithm is composed of two parts, the feed forward one and the feedback one. Influences of the bus voltage and D.C. efficiency are compensated automatically in the feedback algorithm by using a PI algorithm. The stability and robustness of the algorithm is analyzed. Formulas for choosing suitable parameters are given considering both the transient process and the system stability. Experimental results show that, the stable relative error between the target power and the actual output power of the fuel cell system converges to zero. Because of signal noises introduced in the measurements of the current and the voltage, there are some noises in the actual output power. The average relative error (absolute value) during the measuring processes with the closed loop algorithm reduced to1/3 of the level with the open loop algorithm, 1.5% and 4.5%, respectively. Based on the closed loop control algorithm proposed in this paper, a multi-objective optimal energy management strategy considering both of the fuel economy and system durability can be studied in the future.