Purpose: We focused on the comparison of the diagnostic accuracy of preoperative ultrasonography (US) with that of computed tomography (CT), especially for lateral lymph node metastasis (LLNM) in N1b papillary thyroid carcinoma (PTC). Methods: We conducted a retrospective cohort study at the Thyroid Cancer Center of Samsung Medical Center, a tertiary referral center in Korea, by analyzing data collected between January 1997 and June 2015. A total of 1,323 patients who met the following inclusion criteria were enrolled. Results: Diagnostic values were calculated on a “per level” basis by comparing the results of US, CT, and histopathology. Between US and CT, there was no significant difference in the sensitivity (59.3% vs. 61.0%, P=0.073), specificity (83.0% vs. 81.1%, P=0.051), or accuracy (68.6% vs. 68.9%, P=0.660). However, US+CT showed significantly higher sensitivity (74.6% vs. 59.3%, P<0.001) and accuracy (74.0% vs. 68.6%, P<0.001) compared to US alone. Conclusion: There was no significant difference in sensitivity, specificity, and diagnostic accuracy for LLNM between US and CT; however, a combination of CT and US had significantly higher sensitivity, specificity, and diagnostic accuracy in detection of LLNM compared to US or CT alone. We conclude that the strategy for the examination of the lateral neck in PTC patients can be effectively determined by the combination of CT and US rather than US or CT only.