Abstract: The test for NO_x emission from in-use diesel vehicles were conducted based on the lug-down test method regulated in GB3847-2005. The results showed that the concentration of NO_x cannot be easily tested by the lug-down test method due to high fluctuation of pollutant concentration and the limit of instrument response time. Therefore, the lug-down test method was not able to obtain stable emission concentration of NO_x. The test scheme design was adjusted, and the engine dynamometer test was performed. The 100% and 80% of the rated speed were adopted as the test modes of NO_x emission for in-use diesel vehicles. By comparison of three test schemes, Scheme 3 which set two different test modes was reasonable to test the NO_x emission for in-use diesel vehicles. The two test modes were under 100% and 80% of the rated speed respectively, with the operating time of 12 seconds for both modes.