Smoke is the most obvious part of the exhaust emitted from Diesel engines. The methods for smoke testing currently used in Taiwan are the rapid acceleration test under no load and the constant speed test under full load. The smoke concentration is measured with the light reflection method. According to the test procedure, the smoke meter sampling time in the rapid acceleration test is 1∼2 seconds. The current range in sampling time may cause discrepancies in test results. In these tests, free acceleration of engine is achieved by actuating the pedal rapidly. However, the speed of engine acceleration has not been defined clearly in the test procedure. The objective of this research is to study the effect of the operating conditions upon the results of smoke tests.Experiments were carried out in this study. The results of measurements showed that the opacity of the engine exhaust increased rapidly after a delay of about 0.3 seconds and then fell. The entire period of smoke variation was about 1 second. Because the extraction rate of the smoke meter was not uniform, the smoke particles contained in the sampled exhaust may have been affected by changes in the sampling time. It was found in this study that if the timing was retarded 0.3 seconds, concentration of smoke in the sampled gas increased 8%. As for the effect of pedal actuation speed, results showed that the faster the pedal speed, the higher the smoke level. However, the relationship between the smoke level and the pedal speed is nonlinear. The graphic representation is “S” shaped.A smoke transport model was developed in this research to study the effect of exhaust system configuration on smoke measurements. It was found that under a constant engine speed condition, a straight pipe has a delay effect and the muffler has a mixing effect on smoke transport. In the case of engine acceleration, smoke transported along a straight pipe is compressed, but the peak concentration remains the same. The effects of a muffler on smoke measurements are similar to those under a constant speed condition.