Introduction: Renal cell carcinoma is observed to be the most common primary tumour affecting the renal cells that constitute 2.5 – 3% of all the neoplastic problems. In some patients affected with renal cell carcinoma, the major involvement of the renal veins and inferior vena cava by that tumour thrombus comprises in almost 21 – 35% of the cases and thereby reaching up to the major organs like heart. The purpose of this study was to compare the sensitivity of CT scan and colour flow Doppler ultrasound in detecting tumour thrombus in renal vein and inferior vena cava in cases of renal cell carcinoma so that it can be used as a complementary investigation. Materials and Methodology: All patients reported with renal cell carcinoma were included in the study after consent. Patients with previous history of DVT, advanced renal tumours, sensitivity to contrast media for CT scan and those unfit for surgery due to co-morbidities were relatively excluded from the study. The data was collected on structured proforma. Patients’ demographic data, results of investigations and intra-operative findings were entered in proforma in each case. All the data collected were than analysed with the aid of SPSS v12. Results: A total of 45 patients with renal cell carcinoma were evaluated. There were 30 (66.6%) male and 15 (33.4%) females with male to female ratio of 2:1. The age range was 25–70 years with mean age of 56. The tumour involved right side in 27 (60%) cases and left side in 18 (40%) cases. It has been identified that almost 19 (43.3%) of 45 renal veins to be observed with tumour thrombus extension through surgical and pathological findings. 12 cases (44%) out of the 27 cases with right renal veins and 7 (41.6%) out of 18 left renal veins had seen with tumour thrombus extension. Conclusion: Colour Doppler sonography seems to be accurate in detecting the tumour thrombus extension into renal veins and inferior vena cava in patients with renal cell carcinoma.