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Based on the data of 1950 - 1999 monthly global SST from Hadley Center. NCAR/NCEP reanalysis data and rainfall over 160 weather stations in China,investigation is conducted into the difference of summer rainfall in China (hereafter referred to as the "CS rainfall") between the years with the Indian Ocean Dipole (IOD) occurring independently and those with IOD occurring along with ENSO so as to study the effects of El Ni(n)o - Southern Oscillation (ENSO) on the relationship between IOD and the CS rainfall. It is shown that CS rainfall will be more than normal in South China (centered in Hunan province) in the years of positive IOI) occurring independently; the CS rainfall will be less (more) than normal in North China (Southeast China) in the years of positive IOI) occurring together with ENSO. The effect of ENSO is offsetting (enhancing) the relationship between IOD and summer rainfall in Southwest China,the region joining the Yangtze River basin with the ttuaihe River basin (hereafter referred to as the "Yangtze-Huaihe basin") and North China (Southeast China). The circulation field is also examined for preliminary causes of such an influence.