The purpose of this study is to devise a quality control test method by setting the baseline value and acceptable error range used in the CT number linearity test. The CT scan of the AAPM CT performance phantom's CT number linearity insert was performed 10 times with the effective energy of the X-ray beam for each indicated tube voltage. In addition, the average CT number of each pin and water was measured in the reconstructed CT slice images. The linear attenuation coefficient of each pin and water for each effective energy corresponding to the indicated tube voltage was calculated by multiplying the NIST mass attenuation coefficient data by the density. A linear equation was obtained by performing linear matching on the measured average CT number and line attenuation coefficient of each fin and water for each effective energy corresponding to the indicated tube voltage. Based on the obtained linear equation for each effective energy, the baseline value and acceptable error range of the pass criteria of the CT number linearity quality control test were set. In conclusion, it is thought that the baseline value and acceptable error range of pass criteria of the CT number linearity quality control test established from this study will be usefully used for the CT number linearity quality control test in the clinic.