The relationship between the sugar and vitamin C contents in spinach (Spinacia oleracea L) and komatsuna (Brassica campestris L) and the maximum, minimum and mean air temperatures during the cultivation period was analyzed under conditions of limited solar radiation in winter. The results indicated that the minimum and mean air temperatures 10 days before harvest greatly affect the sugar and vitamin C contents in both crops. The sugar content (less than 1 g/100g FW) and vitamin C content (less than 50 - 70 mg/100g FW) in the two crops were not changed when the minimum air temperature 10 days before harvest was above 5 deg C. However, when the minimum air temperature decreased below 5 deg C, the two compounds increased linearly with the decreasing temperature. For example, when the minimum air temperature fell from 5 to -5 deg C, the sugar content increased linearly from approximately 1 to 5 g/100g FW and the vitamin C content increased from 50 - 70 to 175 mg/100g FW. These data show that the sugar and vitamin C contents of spinach and komatsuna can be increased by controlling the minimum air temperature at 5 deg C 10 days before harvesting.