Abstract： Objective To detect the best phase and best tube voltage for the diagnosis of small (diameter ≤ 3 cm) clear cell renal cell carcinoma with dual-energy dual-phase CT.Methods Image manifestations of 27 patients with small (diameter ≤ 3 cm) ccRCCs confirmed by pathology were retrospectively analyzed.All subjects underwent dual-energy biphase (early corticomedullary and delayed phase) scan preoperatively.Two senior radiologists analyzed the images in consensus.The definition of images in different phases and with different tube voltage was classified into 4 levels and was compared by Wilcoxon and Friedman test.The attenuation of the lesions and the adjacent renal parenchyma,the SD value of the anterior abdomen fat were measured.The contrast noise ratio (CNR),lesion kidney ratio (LKR) and the early corticomedullary phase and delayed phase value were calculated.They were all compared with oneway ANOVA.Results The score of definition of lesions in early corticomedullary phase at 80 kV,140 kV and average-weighted 120 kV were 3.30 ± 0.87,2.81 ± 0.92 and 3.11 ± 0.85,respectively,which in delayed phase were 3.70 ±0.54,3.30 ±0.82 and 3.52 ±0.64,respectively.Definition of lesions was better in delayed phase than that in early corticomedullary phase (Z =-2.296,-2.446 and-2.392,respectively; P ＜ 0.05).Either in early corticomedullary phase or in delayed phase,CT value,noise value and CNR were the highest on 80 kV images,which were(302 ± 80)HU,(16.2 ± 2.2) and (4.1 ± 3.4) in corticomedullary phase and (152 ± 31) HU,(16.4 ± 2.7) HU,and (4.7 ± 1.7) in delayed phase.The change of lesion attenuation between early corticomedullary phase and delayed phase on 80 kV,140 kV and averageweighted 120 kV images were (150 ± 76),(72 ± 33) and (96 ± 46) HU,respectively.There was significant difference among the three groups (F =4.541,P ＜ 0.01).Conclusions Delayed phase scan is in favor of small clear cell renal cell carcinoma display.80 kV images are the best for detecting and qualitation of small clear cell renal cell carcinoma when compared with 140 kV and the average-weighted 120 kV images.