In this paper,the effectiveness of accumulated temperatures in the western mountain areas and eastern areas of China over the sub-tropical regions is intercompared. The results show that the ineffective accumulated temperature appeas less in the western mountain areas but effective accumulated temperature occurs more frequently than or equal to those in the eastern areas.The period of day temperature in the western area is more suitable for growing of sub-tropical flora.The equiva-lantly accumulated temperature is 500--1000℃ higher than the actualone in the western mountain areas.Finally, the index of demarcating line of sub-tropical northwest areas is given.