The objective of this study is to investigate the energy consumption of pre-treating excess sewage sludge with microwave irradiation using several parameters, including temperature rise, degree of cell destruction, SCOD/TCOD ratio (solids solubilization), and biogas production to evaluate the energy efficiency. It was found that water content was the most important factor that influenced the energy efficiency of raising the temperature and promoting the solubilization of solid materials. Increasing specific energy (E(s)) accelerated the biogas production, but there was a limit to this process. For quantitative comparison to the energy efficiency of different pre-treatment steps, an empirical method was also proposed based on the experimental data. (C) 2010 Elsevier Ltd. All rights reserved.