Numerical models of three kinds of mass emission measurement systems, i.e. the constant volume sampler (CVS) system, the mini-diluter system and the direct modal-mass measurement system have been built on PC using a software called Mathematica®. The models are capable of simulating gas compounds concentration in the CVS bags and mass emitted during a test, using the time trend exhaust emission patterns, the exhaust gas flow rate pattern, and initial setting values like dilution ratio. Major error factors in the measurement systems, such as H2O condensation, gas compounds present in ambient air, delay and smoothing of the gas stream, and performance of the analyzers, can also be introduced to the calculation. Using the models, various techniques to optimize the sampling system are quantitatively compared.