The water/oil emulsified fuel consists of base fuel oil and water doped with or without a trace content of surface active agents. In this study, the water/oil emulsified fuel was successfully produced without usage of any surface active agents. An experimental study has been implemented in an industrial steam boiler to investigate the combustion characteristics of this emulsified fuel by analyzing the exhaust gas emissions and the thermal efficiency of the boiler. The experiment demonstrated that the water/oil emulsified fuel is effective for NOx and soot emission suppression as well as thermal efficiency improvement. These effects were also demonstrated for the commercially operated water heating boiler. (C) 2011 Elsevier Ltd. All rights reserved.