When measuring the partial pressure of residual gases with a QMS, every gas component in the system must be calibrated. The calibration contains two items: the fragment pattern coefficients and sensitivity. We have measured the fragment pattern coefficients of several samples, such as H2, N2, air, NH3, CO, CO2, O2, Ar, ethylene, propylene, alcohol, acetone, etc. The coefficients are the average value of five data points measured over a long period (from several months to one year). Several factors which limit the accuracy of fragment pattern coefficients of the QMS are discussed. It has been shown that contamination and the geometric accuracy of the analyzer are two most important factors. Under controlled conditions, the reproducibility of fragment pattern coefficients of QMS is better than ±15%.