Diabetic cataract is a major complication of diabetes mellitus. Excess accumulation of sorbitol plays an important role in the pathogenesis of diabetic complications such as cataract formation. In this study, we investigated the inhibitory effect of the extract of the aerial parts of Aster koraiensis (AK) on diabetic cataractogenesis. To examine this further, we evaluated sorbitol accumulation during cataract development using streptozotocin-induced diabetic rat, an animal model of type 1 diabetes. Diabetic rats were treated orally with AK (100 mg/kg and 200 mg/kg body weight) once a day orally for 9 weeks. In vehicle-treated diabetic rats, lens opacity was increased, and lens fiber swelling and membrane rupture were observed. In addition, sorbitol accumulation in diabetic lens was markedly enhanced. However, AK treatment delayed the progression of diabetic cataract through the inhibition of sorbitol accumulation, and prevented lens fiber degeneration in a dose-dependent manner. These observations suggest that AK treatment can delay the progression of lens opacification in the diabetic rats during the early diabetic cataractogenesis.