Diabetes mellitus (DM) is a kind of metabolic disorder characterized by chronic hyperglycemia and glucose intolerance due to absolute or relative lack of insulin, leading to chronic damage of vasculature within various organ systems. These detrimental effects on the vascular networks will result in the development of various diseases associated with microvascular injury. Modern optical imaging techniques provide essential tools for accurate evaluation of the structural and functional changes of blood vessels down to capillaries level, which can offer valuable insight on understanding the development of DM-associated complications and design of targeted therapy. This review will briefly introduce the DM-induced structural and functional alterations of vasculature within different organs such as skin, cerebrum and kidneys, as well as how novel optical imaging techniques facilitate the studies focusing on exploration of these pathological changes of vasculature caused by DM both in-vivo and ex-vivo.