The reasons of continuous heavy rainfall over a large area in South China in June 1998 were analysed using black-body temperature (TBB) data from GMS. The results show that such heavy rainfall is caused by the stronger cold air coming from northern China and frequently dominateds its merger with weaker southwest monsoon: The TBB dekad mean and anomaly fields could directly and quantitatively present the features and causes of the intensity and distribution of the cloud system with heavy rainfall.