This study investigated the influence of aldehyde and hydrocarbon components (HC components) on exhaust odor in DI diesel engines. Aldehyde is an important odorous group in exhaust, and it correlates well with exhaust odor at any engine condition. Formaldehyde (HCHO) in the exhaust has been identified as an important component causing irritating odor.Water-washing of exhaust gases does not trap HC components, while most of the odorous components are trapped with remarkable odor reductions. This indicates that the HC components in the exhaust have no direct effect on exhaust odor. However, the exhaust odor increases with increases in the concentration of the low boiling point HC components. This maybe due to the increase in intermediate odorous compounds like aldehydes, organic acids, or other oxygenated compounds in the combustion condition where the low boiling point HC components increase.The ambient temperature effect on exhaust odor was also investigated and it was found that the odor level increases at the low ambient temperature. At low ambient temperatures, the formaldehyde and the other odorous components in the exhaust are dissolved into the water vapor in the exhaust gas when the water vapor is condensed. It is believed that this water-dissolved formaldehyde and other water-dissolved components are responsible for the higher exhaust odor.