<div class="htmlview paragraph">The accuracy and precision of exhaust gas mass emission measurement has recently come under close scrutiny as the absolute mass emission level has become lower. The uncertainty of the mass emission measurement can not be defined simply as it is a combination of many parameters in the measurement system. This paper lists and reviews the major factors that affect the accuracy in super-low emission measurement when using a constant volume sampler (CVS), such as analyzer performance, HC contamination, excessive dilution of sample gases, error in DF calculations and the variation of vehicle emission itself. Additionally, an alternative sampling system, using the bag mini-diluter (BMD) technique, is investigated in comparison with the enhanced CVS system, i.e. use of dilution air refiner, and heating of the whole system. Some ideas for reducing the HC contamination are also described as it is an important parameter that affects measurement accuracy in both the CVS and BMD systems.</div>