Five different sulfur content fuels were used on a light-duty diesel engine to study the effect of fuel sulfur on emissions. Four regulated emissions: smoke, nitrogen oxide (NO(x)), unburned hydrocarbon (HC) and carbon monoxide (CO) emissions of the engine were investigated, as well as three unregulated emissions: formaldehyde (HCHO), acetaldehyde (MECHO) and sulfur dioxide (SO(2)). The smoke emission decreases continuously and remarkably with the fuel sulfur content, and the fuel sulfur has more influence on smoke emission at lower engine load. The concentration of NO, emissions did not change significantly with the different sulfur content fuels. As the fuel sulfur content decreases, the concentrations of HC and CO emissions have distinct reduction. The HCHO emission values are very low. The MECHO emission decreases with increasing engine load, and it continuously decreases with the fuel sulfur content and it could not be detected at higher engine load with 50 ppm sulfur fuel. The SO(2) emission increases continuously with the engine load, and obviously decreases with the fuel sulfur contents. (C) 2008 Elsevier Ltd. All rights reserved.