In clinical islet transplantation, because the long-term insulin-independence rate is still poor, a method for detailed analysis of the transplanted islets in the liver after transplantation is required. We have established a novel imaging technique suitable for analysis of transplanted islets in liver using an optical projection tomography ( OPT) method. A three-dimensional tomographic image of the transplanted islets in liver was reconstructed. The number of islets transplanted and the number of transplanted islets observed using OPT showed good correlation. The OPT method was used to compare the numbers of transplanted islets in mouse syngeneic and allogeneic transplantation models. Blood glucose concentrations of streptozotocin (STZ)-induced diabetic mice transplanted with syngeneic islets remained normoglycemic and the number of transplanted islets was largely preserved 11 days after transplantation. In mice transplanted with allogeneic islets, hyperglycemia recurred from 7 days after transplantation and the number and the volume of transplanted islets was significantly reduced 11 days after transplantation. These results indicate that OPT imaging and analysis may be a useful tool to quantitatively and sterically evaluation of transplanted islets in liver at the cellular level.