Abstract: Microglia in the retina are resident immunocompetent cells and originate from myeloid lineage. Resting microglia is sensitive in immune surveillance of surrounding circumstance in normal conditions. While in pathological conditions, microglia are activated and  participate in immune and inflammatory reaction. Although activated microglia is involved in infection limitation and tissue repair through phagocytosis and cytokines release, they may also cause tissue damage when overactivated. Microglial activation has been recognized as a neuropathological hallmark in diabetic retinopathy, indicating their important role in the pathogenesis of this disease. This review summarizes biological characteristics and functions of retinal microglia and present knowledge about their activation in the diabetic retinopathy.