Abstract: Aldose reductase (AR) is the key enzyme of the polyol (sorbitol) pathway. The changes of its activity play an important role in the pathogenesis of DR. Increased AR activity is closely linked to the destruction of retinal capillary cells in the ealy stage of DR, and the retinal neovascularization in advanced DR. The mechanisms underlying involvement of increased AR activity in DR are not completely understood. Increased activity of AR promotes the development of DR through a series of biochemical reaction mechanisms, such as the formation of AGE, PKC and MAPK activation, oxidativenitrosative stress, PARP activation and proinflammatory response. New potent AR inhibitor (fidarestat) has been applied, and studies have shown its beneficial effect on diabetic macular edema.