p13 is mitochondrial protein originally identified as one of the proteins down-regulated in pancreatic islets of diabetic mice. Previously, we generated transgenic mice overexpressing p13 in pancreatic beta cells and demonstrated that overexpressed p13 exerts multiple beneficial effects against type 2 diabetes, such as the increase in islets size and insulin secretion. Here, we performed further histological analysis using pancreatic tissue slices of p13-knockout mice in order to investigate the roles of endogenous p13 in pancreatic morphogenesis and function. Although there was no significant difference in islet number, the average islet size was decreased by 42% in p13-knockout mice compared with wild-type mice. Notably, very small islets (<0.001 mm²) were observed specifically in p13-knockout mice. The present results suggest that p13 plays a critical role in islets morphogenesis.