Objective To compare the size discrepancy between ultrasonic and pathological measurement of solitary cN0M0 papillary thyroid microcarcinoma (PTMC), and to explore their correlation with lymph node metastasis. Methods From April 2021 to January 2022, 234 patients with solitary cN0M0 PTMC who received thyroid lobectomy or total thyroidectomy in the Department of Thyroid and Breast Surgery of Nanjing University of Chinese Medicine, Affiliated Hospital of Integrated Traditional Chinese and Western Medicine were analyzed retrospectively. The size discrepancy between ultrasonic and pathological measurement were compared, and the risk factors of central lymph node metastasis were analyzed. Results The mean of maximum diameter of PTMC measured by ultrasound was 6.8 (range 5.6 to 8.4) mm, which was significantly bigger than that measured by pathology 5.0 (range 4.0 to 7.0) mm (P=0.000). Of them, 37.2% of the tumor size measured by ultrasound is consistent with pathology, 61.1% of the tumor size measured by ultrasound is bigger than that measured by pathology, and only 1.7% of the tumor size measured by ultrasound is smaller than that measured by pathology. There was a linear correlation between the diameter measured by ultrasound and pathology. And the regression equation can be expressed as: pathological diameter =0.799 × ultrasonic diameter -0.221. In addition, 28.6% patients had central lymph node metastasis. Multivariate Logistic regression analysis showed that the diameter measured by pathology is a risk factor for central lymph node metastasis in patients (OR=17.845, 95%CI: 2.507-127.025, P=0.004), and the cutoff value is 5.5 mm which corresponded to the diameter measured by ultrasound as 7.2 mm. Conclusions The sizes of solitary cN0M0 PTMC measured by ultrasound and pathology are different but also correlated. PMTC with pathological diameter >5.5 mm with its corresponding ultrasonic diameter as 7.2 mm indicated an increased risk of central lymph node metastasis.