Using rainfall data from the Global Precipitation Climatology Project (GPCP), NOAA extended reconstruction sea surface temperature (ERSST), and NCEP/NCAR reanalysis, this study investigates the interannual variation of summer rainfall southwest of the Indian Peninsula and the northeastern Bay of Bengal associated with ENSO. The composite study indicates a decreased summer rainfall southwest of the Indian Peninsula and an increase in the northeastern Bay of Bengal during the developing phase, but vice versa during the decay phase of El Ni?o. Further regression analysis demonstrates that abnormal rainfall in the above two regions is controlled by different mechanisms. Southwest of the Indian Peninsula, the precipitation anomaly is related to local convection and water vapor flux in the decay phase of El Ni?o. The anomalous cyclone circulation at the lower troposphere helps strengthen rainfall. In the northeastern Bay of Bengal, the anomalous rainfall depends on the strength of the Indian southwest summer monsoon (ISSM). A strong/weak ISSM in the developing/decay phase of El Ni?o can bring more/less water vapor to strengthen/weaken the local summer precipitation.