Uniform design coupled with grey relational analysis approach was used in this study to evaluate the influence of four main factors governing PHB production by activated sludge in a sequencing batch reactor (SBR). Influent chemical oxygen demand (COD), nitrogen and phosphate concentrations and initial pH were considered for process evaluation. First, a mixed-level uniform design methodology was employed to arrange the experiments. Then, a grey relational analysis method was used to investigate the influential priorities of these four factors on PHB yield, production rate and the overall performance. The results indicate that influent COD concentration was the most important factor governing PHB yield, while pH showed stronger impacts on the PHB production rate and the overall PHB production performance. This approach provides an effective way to identify the influential priority of operating factors on PHB production process, and it can also be appropriately extended to other complex biological wastewater treatment processes. (C) 2014 Elsevier B.V. All rights reserved.