Research has been conducted on the effect of variations in X-ray tube voltage to value of Contrast to Noise Ratio (CNR) on CT Scan at Bali Mandara Hospital using a phantom as a patient replacement. This research aims to determine the effect of X-ray tube voltage to the CNR value. Exposure factors used are X-ray tube voltage with variations of 80, 110, 120 and 135 kV, constant X-ray tube current of 150 mA and constant exposure time of 1 s. The readings of Io, Ib, and sb values in phantom images were performed using RadiAnt DICOM Viewer software (64 bit) and analysis of the effect X-ray tube voltage on CNR values was determined by regression test. The results of the analysis show that the variation of the X-ray tube voltage has a significant effect on the CNR value, where the greater X-ray tube voltage, the greater the CNR value. When the X-ray tube voltage is adjusted to 135 kV, the optimal CNR values are 113.52 for air, 35.06 for derlin, 13.93 for acrylic, 10.44 for nylon and 12.19 for polypropylene.