Objective: We aimed to compare the normal main portal vein diameter measured in computed tomography with the commonly used upper limit value. Materials and Method: Computed tomography examinations performed between March 2015 and April 2018 in our department were scanned from the archive system. Mean portal vein diameters were measured onaxial contrast-enhanced and non-enhanced abdominal CT scans of the patients without any known disease. Results: 500 main portal vein measurements were performed from 276 individuals. In the non-enhanced images (n = 243), the mean diameter of main portal vein was 15.03 ± 1.72 mm and in the post-contrast enhanced images (n = 257) the mean diameter of the main portal vein was 15,05 ± 1.71 mm. These values showed a significant difference from the widely accepted upper limit of 13 mm (95% confidence interval for non-enhanced images: 1.81-2.25 mm higher, p <0.001, 95% confidence interval for post-contrast images: 1.84-2.26 mm higher, p <0.001). The mean main portal vein diameter measured from contrast tomography images was 0.26 mm wider than the mean main portal vein diameter measured at non-enhanced images (95% confidence interval: 0.23-0.29 mm, p <0.001). Conclusion: The mean normal portal vein diameter measured in computed tomography (15.05 mm) was significantly higher than the accepted upper limit of 13 mm (p <0.0001). The mean main portal vein diameter in contrast-enhanced tomography was 0.26 mm larger than the mean main portal vein diameter measured in the non-enhanced examination.