Abstract: Using the temperature data of China Meteorological Administration, the ERA reanalysis data as well as Hadley monthly mean SST, we have analyzed the variations of summertime temperature in the middle and lower reaches of the Yangtze River and the associated mechanisms on interdecadal timescales. The summertime temperatures show three principal modes. The interannual and interdecadal variations share the same modes. The anomalous circulations related to both EOF1 and EOF3 show a baroclinic structure in vertical while EOF2 barotropic. It is interesting to notice that PC3 is consistent with the global warming trend; it is warming (cooling) in the lower (the upper and middle) reaches of the Yangtze River. On interdecadal timescale, the aforementioned three modes are related to the different circulation anomalies. The leading mode is found to be related with the wave trains propagating along the Asian jet as well as the wave trains originated from the North Atlantic. And the second mode is connected with the wave trains propagating along the Asian jet as well as the EAP (Eurasion Pattern) trains from the tropical western Pacific showing the coeffects from the middle latitudes and the low latitudes on the region we concern. Besides, the third one correlates with the AO (Arctic Oscillation), implying the interaction between the high latitudes and the middle latitudes. Additionally, the SSTA in the Pacific Ocean as well as the Indian Ocean affects the spatial patterns of the temperature anomalies in the middle and lower reaches of the Yangtze River. The northernsouthern seesaw mode of surface air temperature is related to the PDO, while the easternwestern seesaw mode is related to the abnormal cold SSTA in the northwestern Pacific. The SSTA in the Indian Ocean may also affect the first two modes.