While pancreatic islet transplantation has emerged as one of the most promising therapies for diabetes mellitus, determining the fate of islet grafts remains a challenge. Transplanted islets are damaged by hypoxic stress and immune responses in the period shortly following transplantation. Understanding the mechanisms of islet graft loss is difficult without real-time monitoring in vivo. Recently, nanotechnology has emerged in studies of biomedical imaging. Nanoparticles can enhance the versatility of different imaging modalities. Moreover, nanoparticles provide potential probes to image transplanted islets by optical imaging, magnetic resonance imaging and positron emission tomography. The nanoparticles are delivered to islet grafts using different methods, such as non-specific uptake, conjugation to the islet membranes and targeting to a membrane receptor or transporter. If applied clinically, these methods would provide information about the viability and function of transplanted islets, which will enhance graft survival. This review focuses on recent advances in nanoparticles for in vivo imaging of islet transplantation.