Objective:To explore the application value of ultrasound elastic strain ratio in diagnosing thyroid nodules of TI-RADS 4-6 categories. Methods:Conventional ultrasound was applied to 324 thyroid nodules (from 303 patients) to observe their two-dimensional ultrasonographic characteristics. The 324 thyroid nodules were classified into different groups according to the TI-RADS,and then undergone ultrasonic elastography and the strain ratios of them were recorded. A receiver-operating characteristic curve(ROC)was used to identify the cut-off point in the differential diagnosis of thyroid nodules. The 137 thyroid nodules of TI-RADS 4-6 categories were classified into three groups to analyze the application value of strain ratio in each group:group Ⅰ,TI-RADS 4A category;group Ⅱ,TI-RADS 4B category;group Ⅲ,TI-RADS 5-6 categories (the pathologically confirmed cases in category 6 have been removed). Results:The best boundary value of strain ratio was 0.55. The difference of strain ratio in group I was not statistically significant,while those in group Ⅱ and group Ⅲ were statistically significant. The sensitivity,specificity,accuracy and positive predictive value in group Ⅱand group Ⅲ were 90.9%,75.0%,87.5%,93.0% and 85.7%,80.0%,84.6%,94.7%,respectively. Conclusion:The combination of TI-RADS classification system and strain ratio in group Ⅱ and group Ⅲ was of high application value,which suggests that the malignant nodules should be given early surgical treatment in stead of examination like fine needle aspiration biopsy. While in group I,the application value of strain ratio was limited,thus other examination methods should be used at the same time as early as possible to avoid delays in the diagnosis of nodules.