ABSTRACT Objective To explore the ultrasonic characteristics of Bethesda Ⅲ (atypia of undetermined significance/ follicular lesions of undetermined significance) thyroid nodules，and to observe the value of the American College of Radiology thyroid imaging report and data system (ACR TI-RADS) for benign and malignant Bethesda Ⅲ thyroid nodules. Methods The clinical and ultrasound data of 180 thyroid nodules in 175 patients who underwent thyroid ultrasound and ultrasound-guided FNAC and cytology results of Bethesda Ⅲ and surgery in Affiliated People''s Hospital of Shanxi Medical University from January 2018 to June 2020 were retrospectively analyzed. They were divided into benign group and malignant group according to pathological results. The internal composition

echogenicity

margin

shape

and echogenic foci of benign and malignant nodules were compared

and statistically significant indicators were included in the multivariate logistic regression analysis. Each thyroid nodule was graded by ACR TI-RADS. The sensitivity

specificity

accuracy

positive predictive value

negative predictive value and area under curve (AUC) of ACR TI-RADS for Bethesda Ⅲ thyroid nodules were calculated by ROC curve. Results Of the 180 thyroid nodules

66 cases were benign nodules and 114 cases were malignant nodules. Univariate analysis showed a significant difference in internal composition

echogenicity

margin

shape

and echogenic foci between benign and malignant nodules (all P < 0.05). Multivariate logistic regression analysis showed that taller than wide

hypoechoic

microcalcification and mixed calcification were independent risk factors of malignant nodules (OR = 4.903

4.608

4.458

3.821

all P < 0.05). With the increase of ACR TI-RADS classification

the malignant rate of nodules increases significantly（rs=0.653，P＜0.05）; With ACR TI-RADS 5 as the cut-off point

the sensitivity

specificity

accuracy

positive predictive value and negative predictive value in the diagnosis of Bethesda Ⅲ thyroid nodules are 83.3%

80.3%

82.2%

88.0%

73.6%

respectively

and the area under the curve (AUC) are 84.1%. Conclusion The ultrasonic characteristics of benign and malignant Bethesda Ⅲ thyroid nodules were different

taller than wide

hypoechoic

microcalcification and mixed calcification were independent risk factors of malignant nodules; ACR TI-RADS can be helpful for distinguishing malignant and benign Bethesda Ⅲ thyroid nodules and can guide clinical management.