Diabetic retinopathy is a major cause of blindness all over the world, and it is one of the most serious and common microvascular complications of diabetes. Breakdown of the endothelial blood-retinal barrier(BRB), as occurs in diabetic retinopathy, result in vasogenic edema and neural tissue damage, causing loss of vision. The inner BRB is created by complex tight juctions of retinal capillary endothelial cells, this barrier prevents the free diffusion of substances between the circulating blood and the neural retinal, the inner BRB efficiently supplies nutrients to the retinal and removes endobiotics and xenobiotics from the retina to maintain a constant milieu in the neural retina. The central mechanism of altered inner BRB function is a change in the permeability characteristics of retinal endothelial cells caused by elevated levels of cytokines, growth factors, advanced glycation end products, inflammation, hyperglycema and loss of pericytes. This article reviews the relationship between diabetes and the ultrastructure changes of BRB.