The effects of fuel properties on diesel engine exhaust emission characteristics are investigated using eleven kinds of fuel with varying levels of sulfur and aromatic contents. Exhaust emissions from three engines are measured over the new Japanese 13-mode cycle as well as the U.S. transient test procedure. Engine test results show that reducing the fuel sulfur content decreases particulate levels. This effect is more pronounced for engines that emit more SOF. Enriching the aromatic content with dicyclic and other polycyclic compounds increases particulate, NOx, CO, and THC emissions. This particulate increase is due to the increase of SOF. Accordingly, low sulfur fuel should be produced without increasing the aromatic content, otherwise the SOF increase will offset the particulate reduction effect of the low sulfur fuel.The demand for diesel engines with reduced exhaust emissions has become stronger, as countries try to cope with global environmental problems and the worsening big city conditions. The governments of the U.S., Europe, and Japan are strengthening exhaust emission regulations, especially for NOx and particulate emissions. However, the well-known trade-off between these two emissions makes it difficult to achieve significant reductions in both with only engine modifications. Consequently, research has been conducted to clarify the effects of fuel properties on diesel exhaust emissions. For example, fuel sulfur content has been reduced in order to reduce the acid rain problem, and a further reduction of up to 0.05wt% is planned in order to reduce particulate levels. These types of fuel property improvements require the oil industry to change their refinery facilities, which will lead to further fuel property improvements, such as in sulfur content, aromatic content, the cetane index, and distillation.Many researchers have discussed the effects of fuel property changes on diesel exhaust emissions (1, 2, 3, 4, 5, 6, 7, and 8)*. The effects of each fuel property on exhaust emissions cannot be easily ascertained, however, because of their complex interactions. It is therefore important to accumulate data on the effects of fuel property changes on diesel exhaust emissions and to study the data from various viewpoints in order to improve fuel quality. Hence, we have investigated the effects of fuel sulfur and aromatic contents on exhaust emission characteristics of heavy-duty diesel engines.