Although natural gas can be used in diesel engines, it causes poor performance and emissions at some engine loads. This study proposes using natural gas in a diesel engine as a dual fuel, and investigates the effects of employing diethylether (DEE) as an additive for eliminating some drawbacks of natural gas. The test fuels used in this study were diesel fuel, standard dual fuel containing 40% natural gas and dual fuels containing DEE as an additive. DEE was blended with diesel fuel, which was used as a pilot fuel, at the volumetric ratios of 5% and 10%. It has been determined that the use of the standard dual fuel causes poorer performance at low and medium loads. When compared with diesel fuel, the use of dual fuel yields higher CO and HC emissions at all loads along with lower NO emissions expect for high loads. Furthermore, the use of DEE as an additive leads to an improvement in brake thermal efficiency and specific energy consumption, while causing lower CO and NO emissions in comparison to the use of standard dual fuel. (C) 2013 Elsevier Ltd. All rights reserved.