Relationship between summer rainfall over the eastern part of China (east of 110¿E) and East Asian Summer Monsoon (EASM) is studied based on summer rainfall coded levels data set for the period of 1470--1999, rain gauge data set for the period of 1951--1999 and sea level pressure (SLP) for the period of 1873--1997. The distinct 80 yr oscillation of summer rainfall is found over North China (near 35¿--40¿N and east of 110¿E), southern part of Northeast China (south of 42¿N in Northeast China), lower-middle Yangtse River valley (east of 110¿E along Yangtse River) and South China (south of 25¿N and east of 110¿E). This oscillation over North China is in phase with that over South China and out of phase to that along lower-middle Yangtse River valley. Summer rainfall over North China correlates negatively with averaged SLP for the area near (105¿--115¿E and 30¿--35¿N), but positively with averaged SLP for the area near (120¿--130¿E and 20¿--25¿N). Therefore, an index of EASM is defined using the difference of averaged SLP over the above two regions. Summer rainfall over North China is greater than normal and drought occurs along lower-middle Yangtse River valley when strong EASM reaches North China. On the contrary, drought is found over North China and flood appears along lower-middle Yangtse River valley when EASM is close to normal or weak. Finally, the interdecadal variability of EASM is studied using long-term summer rainfall coded levels data set over North China during the past 530 yr.