The engine brake is widely used as auxiliary brake device for its continuous brake torque. The engine brake performance is usually determined in the laboratory or proving ground according to relevant standards. The main purpose of this paper is to introduce an on-vehicle measurement system to measure the engine brake performance in the driving process. The on-vehicle measurement system makes use of the vehicle driving information to deduce the engine brake performance during transportation, which can reduce the test times in the laboratory or proving ground and the sensor cost. The measurement system based on the vehicle longitudinal dynamics can adapt to various vehicle automatically without measuring the vehicle or engine parameters before installation. And the measurement system can also estimate gross vehicle mass approximately. First the vehicle state information from the controller area network and the road gradient from the gradient sensor are used to compute gross vehicle mass and relevant driving resistance coefficients by the vehicle longitudinal dynamics in the driving process. And then the data fusion algorithm with simple structure and fast computation speed is built to process these computation results for accurate parameters. Finally the relationship between the engine braking torque and the engine speed can be deduced by the above processed computation results in the gear taxiing process. Some vehicle tests are carried out to validate the effectiveness of on-vehicle engine brake performance measurement system. Percent error range of the engine braking torque measurement is 3.6% to 6.7% within the normal working speed range of the engine brake 1600 r/min to 2200 r/min. And the measurement error of gross vehicle mass is 7.92%. The errors of the engine brake measurement system can satisfy the general requirements of the researches relevant to the engine brake performance. The engine brake performance test will not be limited to experimental places and the performance test cost will also be reduced when the on-vehicle measurement system is popularized.