Hepatic arterial infusion chemotherapy (HAIC) is one of the approaches used to treat advanced hepatocellular carcinoma (HCC). Here, we describe 2 cases involving unexpected tumor necrosis after interventional alteration of the hepatic arterial flow during implantation of a port-catheter system for HAIC. Case 1 involved a 42-year-old man with diffuse HCC accompanied by a tumor thrombus in the main trunk of the portal vein. After the right hepatic artery (RHA) derived from the superior mesenteric artery (SMA) was occluded by coils, a port-catheter system was successfully implanted using the gastroduodenal artery (GDA) coil method. The next day, he developed a fever and had right upper abdominal pain. A marked increase in transaminase and lactate dehydrogenase levels was observed. Contrast-enhanced computed tomography (CT) showed tumor necrosis in both the parenchymal tumor and portal vein tumor thrombus. Case 2 involved a 62-year-old man diagnosed with a large HCC located in segments VII and VIII of the liver and abdominal lymph node metastasis. As in case 1, angiography revealed the RHA branched from the SMA. After the replaced RHA and right gastric artery were embolized with coils, a port-catheter system was successfully implanted. Although he showed neither clinical symptoms nor abnormal laboratory data the next day, contrast-enhanced CT revealed tumor necrosis in a large part of the HCC lesion. In conclusion, careful attention is required in the interventional alteration of hepatic arterial flow for implantation of a port-catheter system for HAIC against advanced HCC.