The prognosis of hepatocellular carcinoma (HCC) patients with tumor thrombus in the main portal branch (PVTT) is very poor and a standard treatment regimen for HCC with PVTT has not yet been established. Therefore, a new strategy is necessary to control the patients of HCC with PVTT. Conventional radiation therapy for HCC was not effective because of low liver tolerance. On the other hand, because three-dimensional conformal radiation therapy (3D-CRT) can focus high-dose irradiation on a small area, we tried it for patients with PVTT as the preoperative adjuvant treatment. 3D-CRT which targeted the PVTT was performed for two HCC patients with PVTT before radical hepatectomy. Treatment schedules of 3D-CRT were as follows: in Case 1, the daily fraction size was 3 Gy, given five times per week, and the total dose was 39 Gy. In Case 2, the daily size was 6 Gy and the total dose was 60 Gy. Three or four weeks after 3D-CRT, the main tumor and PVTT were completely removed. On histological examination, the PVTT showed complete necrosis in Case 1, and 60% necrosis in Case 2. No serious complications occurred, and there have been no recurrences for more than one year after surgery in both cases. 3D-CRT targeting the PVTT may be promising as a preoperative adjuvant therapy for HCC with PVTT.