In the summers of 1998 and 2010, severe °oods occurred in the middle and lower reaches of the Yangtze River. Although an El Ni~no event took place preceding each of the summer °oods, signi¯cant di®erences between the two summer °oods and the two El Ni~no events were identi¯ed. The 1997/98 El Ni~no is a conventional one with strongest warming in the central-eastern Paci¯c, whereas the 2009/10 event is an El Ni~no Modoki with strongest warming in the central Paci¯c. In this study, summer rainfall anomalies (SRA) in the two years were ¯rst compared based on the rainfall data at 160 stations in mainland China, and a signi¯cant di®erence in SRA was found. To understand the underlying mechanism for the di®erence, the atmospheric circulation systems, particularly the western North Paci¯c anticyclone (WNPAC), the western Paci¯c subtropical high (WPSH), and the low-level air °ows, were compared in the two years by using the NCEP/NCAR reanalysis data. The results display that the WNPAC was stronger in 2010 than in 1998, along with a northwestward shift, causing weakened southwesterly from the Bay of Bengal to the South China Sea but intensi¯ed southerly in eastern China. This resulted in less water vapor transport from the tropical Indian Ocean and the South China Sea but more from the subtropical western Paci¯c to East Asia. Subsequently, the rainband in 2010 shifted northward. The di®erence in the WNPAC was caused by the anomalous ascending motion associated with the warming location in the two El Ni~no events. Furthermore, the role of tropical sea surface temperature (SST) in modulating these di®erences was investigated by conducting sensitivity experiments using GFDL AM2.1 (Geophysical Fluid Dynamics Laboratory Atmospheric Model). Two experiments were performed, one with the observed monthly SST and the other with June SST persisting through the whole summer. The results suggest that the model well reproduced the primary di®erences in the atmospheric circulation systems in the two years. It is found that the di®erence in El Ni~no events has shaped the rainfall patterns in the two years of 1998 and 2010. At last, the case of 2010 was compared with the composite of historical El Ni~no Modoki events, and the results indicate that the impact of El Ni~no Modoki varies from case to case and is more complicated than previously revealed.