Duplex kidney is a common congenital malformation appeared as duplication of pelvis and ureter. However, renal duplication within sinus renalis is an extremely rare variation of the renal collecting system. In this study, we report a case of an asymptomatic kidney disease in a 33-year-old man, who demonstrates abnormal echo of renal sinus anomaly discovered incidentally in ultrasound examination. Computed tomography urography (CTU) exhibited the other small duplex kidney located in renal sinus. In the excretory phase images, the contrast medium within its small renal pelvis could be seen to flow into the right major renal calices. This case exhibited a very rare anatomical variation of duplicated renal malformation.