We acquired NOx emission data, etc. by carrying out chassis dynamometer tests and road tests with using the sensor-based emission measurement system (SEMS) consisting of the NOx sensor, the digital tachograph and the gas flow meter. Based on these tests, we examined NOx emission evaluation method of a heavy-duty truck driving on road. Consequently, it was found that the indication of new proposal “NOx emission (g) per CO₂ 1 kg” has the advantage of canceling the measurement error of the instantaneous exhaust gas flow, furthermore the relationship between this new indication and the engine workload-based NOx emission (g/kWh) is strong correlative.