Power system stabilizers (PSS) are used to damp out the low frequency power system oscillations caused by sudden change in the load, loosing of one generator or losing a transmission line. In this paper, an evolutionary algorithm known as biogeography-based optimization (BBO) is used to find the optimal settings of the power system stabilizer (PSS) parameters for a single machine infinite bus system operating at different loading conditions and the results are compared with a conventionally designed power system stabilizer to show the effectiveness of the proposed algorithm.