objective To compare the accuracy of ultrasonography(US) and acoustic shadow on measuring the size of kidney stone with non-enhanced CT measurement as the "gold standard", and evaluate the feasibility of measuring kidney stone size by posterior acoustic shadow width Methods The US report, US image and non-enhanced CT image of 51 kidney stones in 49 patients were analyzed retrospectively.The relationship between US stone size, shadow size and CT stone size was compared. Results A total of fifty-one stones , the average CT stone size was (8.3±4.6) mm, and the average US stone size was (10.1± 4.3) mm. US stone size overestimated CT stone size with a mean bias of （1.8±2.0）mm ,the mean bias of stones≤5 mm was the largest (2.5±1.6) mm .The difference between US size and CT size was statistically significant for stones ≤10 mm（P＜0.05）,however, there was no statistically significant difference between the US size and CT size for stones >10mm (P=0.289). Fourteen stones without obvious shadow ,of which nine(64.3%) stones were ≤5 mm，thirty-seven stones demonstrated a posterior acoustic shadow, for stones≤10 mm, the shadow size was closer to the CT size than the US size, and the difference was statistically significant (P<0.05). Conclusion directly measuring the hyperechoic width of the stone by US can overestimate the size compared to CT, especially for small calculi. Measurement of posterior acoustic shadow width of calculi is valuable for improving the accuracy of ultrasonic measurement of calculi size, but it needs further confirmation.