In patients that have undergone liver transplants, a postoperative reduction in the blood flow of the liver graft represents a critical complication. We recently encountered an interesting phenomenon; that is, we found that the rSO2 level of the liver graft, as measured by NIRS, drops in patients that subsequently require an emergency liver biopsy. An 8-month-old female and an 8-month-old male underwent living donor liver transplants for biliary atresia. In both cases, a reduction in rSO2 was detected before an emergency liver biopsy was required. As a result of biopsy examinations, both patients were diagnosed with acute graft rejection. NIRS might be useful for graft management during the postoperative period in pediatric patients that undergo liver transplantation. After a liver transplant, a reduction in the rSO2 of the graft might be indicative of the onset of vascular complications.