Abstract: Based on the chemical reaction of fuel with air in internal combustion engine and the elementary equilibrium before and after combustion, the relationship of the quantity of total exhaust gas with the fuel consumption and with the quantity of each exhaust composition was analyzed, and a new model/method was set up for measuring the smoke emission under on road conditions. The precision of the model/method was validated by an engine bench test. The smoke emission of a diesel vehicle running on road was measured using this method.