Objective: To explore diagnostic value of anteroposterior-to-transverse (A/T) ratio predicting thyroid cancer. Methods: 2306 nodules were divided into 5 groups according to their size, and performance of A/T ratio predicting thyroid cancer was analyzed in each group on the basis of the sensitivity, specifity, accuracy and Youden index. Results: A/T ratio got the median of 0.83 and interquartile range of 0.28. The area under the receiver operating characteristic curve (AUC) was 0.709(P<0.001). When the diameter of a thyroid nodule was <1.5 cm, A/T>0.9 was associated with higher sensitivity, accuracy, and Youden index than A/T>1.0, but the specificity was lower. When the diameter of a thyroid nodule was 21.5 cm, A/T>0.9 got higher sensitivity and Youden index, whereas the specificity and accuracy were lower, with comparison with A/T>1.0. Conclusion: The A/T ratio was a meaningful indicator of thyroid cancer, and its predictive effectiveness could be influenced by nodule size.