The cabbage aphid Brevicoryne brassicae (L.) (Hemiptera: Aphididae), one of the most important pests of cruciferous crops, overwinters as nymph and adult on winter cabbages in Tehran, Iran, and forms large populations on these plants during winter. To determine the cold hardiness of this aphid, adults and first-instar nymphs were collected monthly from ornamental cabbages planted in the field from October of 2009 to May of 2010. Supercooling points and LT₅₀ values (the temperature at which 50% of population died after 24-h exposure to subzero temperatures) were investigated. To elucidate the relation between cold hardiness and cryoprotectants, sugars and polyols were determined by high performance liquid chromatography. Changes of mean supercooling point were small during sampling dates. LT₅₀ values of adults decreased gradually from -6.9°C in October to -12.2°C in January, showing the increase in cold hardiness during cold seasons. Moreover, nymphs showed similar trends in LT₅₀ values. Glucose, mannitol, myo-inositol, and trehalose were identified in overwintering aphids. Total amount of the cryoprotectants increased to the highest (72.8 ± 9.2 µmol/g fresh weight) in January. There was a significant correlation between LT₅₀ values and the cryoprotectant contents, suggesting their important role in aphid cold hardiness. Results have shown that B. brassicae is sufficiently cold tolerant to survive mild winter temperatures and is able to form large populations on winter cabbages, causing it to be considered a threat to early season crops, especially oilseed rape.