Abstract: Objective: To investigate the effect of different size of Region of interest(ROI) on CT value and material decomposition measurement in the standard water phantom using single-source dual-energy CT. Methods: The standard water phantom provided by GE company was scanned by single-source dual-energy CT machine(Discovery CT 750 HD, America GE Company). The image processing was performed on GSI Viewer browser to acquire and record CT value(HU), the concentration value of water-iodine(mg/cm³). The scanning parameters were as follows: scanning visual field: medium, rotating speed of 0.8 seconds/rotation, pitch 0.984, detector width 40 mm, current: 550 m A. The two drawn ROIs were round, located in the central part of the water phantom. Their areas were(600 ± 30) mm² and(3 000 ± 30) mm², and then recorded as ROI1 and ROI2 respectively. Paired sample t test was used for statistical analysis. Results: ROI1 CT value was(2.45 ± 1.83) HU, ROI2 CT value was(2.55 ± 1.68) HU, there was no statistical significance between CT value of ROI1 and ROI2(P = 0.06); The water-iodine value was(1 001.22 ± 0.86) mg/cm³ in ROI1, and that of ROI2 was(1 001.31 ± 0.84) mg/cm³, there was no statistical significance between water iodine value of ROI1 and ROI2(P = 0.135). Conclusions: Different ROI size will have no effect on the measurement of CT value and the concentration of water-iodine from single-source dual energy CT.