Purpose <p>This study aims to introduce a new method for measuring phantom tilt and to improve the accuracy of gantry tilt measurement with a correction of the measured phantom tilt on an axial image of the cylindrical phantom in computed tomography (CT) imaging.</p> Method <p>A GE Revolution EVO CT scanner and a GE phantom were used. The study was conducted with gantry tilt setting variations ranging from 0° to 30°, and phantom tilt settings of 0° and 5°. The phantom tilt was measured based on changes in the center coordinates of phantom images from two different locations along the z-axis, while the gantry tilt was determined from the ratio of lateral (LAT) and anterior-posterior (AP) diameters. A correction was performed by subtracting the phantom tilt angle from the measured gantry tilt. The analysis included paired t-tests, linear regression, and its comparison against a gantry tolerance threshold of ≤ 3°.</p> Results <p>The measured phantom tilt was less than 1.5^o compared to the phantom tilt setting. The results showed that phantom tilt correction significantly improved the accuracy of the measured gantry tilt. The average difference of the measured and set gantry tilt was 3.46° (without phantom tilt correction) and 0.87° (with phantom tilt correction) for a phantom tilt setting of 5°.</p> Conclusion <p>This phantom tilt correction effectively increases the accuracy of the gantry tilt measurements on an axial CT image of any cylindrical phantom.</p>