For a higher sensitivity, the parameters of liquid chromatography-mass spectrometry-selected ion monitoring (LC-MS-SIM) using atmospheric pressure chemical ionization (APCI) were examined. The scan-range means the range of atomic mass unit which was scanned at the center of an established value. The sampling-point-number means the number of measurement points within the scan-range. The counting-number means the number of the data acquisition per measurement point. The area and height of a peak as sensitivity, the full width half maximum (FWHM) and the actual scan-time which was required for the scanning were measured. The plant hormone, indole-3-acetic acid (IAA) was used as a standard chemical. The suitable parameters for higher sensitivity on the measurement limit level were as follows: 1) The scan-time might correspond to the value of FWHM. 2) The counting-number was over about 7,000. 3) The scan-range might correspond to a decimal fraction of the molecular weight of the sample substance. The results showed that the value obtained from repeated sampling-point-number was a total amount, that the value obtained from repeated counting-number was a mean and that there were no loss of time on repetitions of the sampling-point-number and the counting-number.