Islet transplantation is a cell replacement therapy to improve glycometabolic control in type 1 diabetic patients. Establishing methods to monitor engrafted islets, as well as the islet preparation, is important when performing islet transplantation. Since current imaging techniques are still not available to directly detect transplanted islets, we propose a novel method to visualize transplanted islets using high-frequency ultrasound (HF-US), and to evaluate the correlation between these US findings and metabolic parameters. We transplanted syngeneic (BALB/c mice) and xenogeneic (SD rats) islets into the renal subcapsular space of diabetic mice. The recipient mice were examined by HF-US until post-operative day (POD) 28 and, while syngeneic islets could be detected by HF-US throughout the observational period, the xenogeneic islets had vanished by POD 28. The islet volume calculated by HF-US was correlated with the number of transplanted islets (R(2) = 0.31, p = 0.0003) and the metabolic function of islets (blood glucose: R(2) = 0.15, p < 0.0001, serum insulin: R(2) = 0.22, p < 0.0001). In conclusion, HF-US may be a useful imaging modality for visualizing the islet mass in renal subcapsular transplantation models. It may also be an available modality for clinical settings in the future.