Abstract: High performance liquid chromatography-tandem triple quadrupole mass spectrometry（LC-MS/MS） instrument influences on the measurement results of isotope dilution mass spectrometry method were studied with the measurement of chloramphenicol in milk powder. The basic parameters of API5000, Q-trap5500, Agilent6490, Waters XEVO TQS were investigated e.g. linearity, sensitivity and precision. The results showed that the linearity and sensitivity of the four instruments were good and similar each other, but the precision was quite different with the relative standard deviations of 1.0%—9.8% at the concentration range of 0.2—1.0 ng/g. The affection of instruments was obviously observed in the level of lower concentration. Matrix effect of different instruments during the measurements process was studied. The response of the analyte and its isotope diluent were different in all of mass spectrometers. And the disparity increased with the growing of the concentration of analyte. The use of different apparatus might cause different and even opposite matrix effects such as matrix inhibition and enhancement. And the differences of the effects could be significant. Therefore, the instruments may be play a very important role during the detection programs using LC-MS/MS. The impact of the LC-MS/MS instrument on the measurement results should be given full consideration in the application of isotope dilution mass spectrometry.