Abstract: To understand the NO_x emission characteristics of China Ⅵ heavy-duty diesel vehicles at different altitudes, a portable emission measurement system (PEMS) was used to conduct real driving emission tests on China Ⅵ heavy-duty diesel vehicles in four cities with different altitudes (including Xiangyang, Kunming, Lijiang and Shangri-la). This study showed that with the increase of altitude, NO_x emission showed an increasing trend, and the average NO_x emission rate under high altitude was 4.65-20.58 times that of the plain, and NO_x emission factor was 2.80-13.75 times that of the plain. NO_x emission factors of Shangri-la were 1.27-13.75 times higher than those of Xiangyang, Kunming and Lijiang under different load conditions. NO_x emission factors of urban road were 1.05-6.49 times higher than those of suburban road and freeway, and the emission factors of urban road in Shangri-la exceeded 400 mg/km. In the interval of Bin 11-Bin 14 and Bin 21-Bin 28, NO_x emission rates showed a trend of increases followed by decreases with the increase of VSP. NO_x instantaneous emission rate peaked when the vehicles drove from urban road to suburban road and suburban road to freeway at different altitudes. The high NO_x emission area was concentrated in the high speed and high torque range. The coefficient of determination between altitude and the average NO_x emission factor was 0.86, which showed a fair positive correlation.