This study aims to find the optimum threshold for the automatic measurement of slice thickness using ACR CT accreditation phantom. The ACR CT accreditation phantom was scanned using Siemens Somatom Perspective CT scanner. The nominal slice thicknesses of 1.5, 3, 5, 6, 7, and 10 mm were investigated. Our automated method was developed to obtain accurate slice thickness values. Several threshold values from 0.10 to 0.50 with increment of 0.05 to find optimum value were investigated. The results obtained from each threshold were then compared with the nominal slice thickness to determine the optimal threshold value. It is found that the optimum threshold in the automatic measurement of slice thickness with nominal slice thickness values from 1.5 to 10.0 mm is from 0.35 to 0.40. Using this range, the different between the nominal slice thickness and measured slice thickness is within 0.5 mm. The optimal threshold for automatic slice thickness measurement has been determined. The optimal threshold would lead to more accurately automated slice thickness measurement.