Abstract: An index of land-sea thermal difference (ILSTD) has been designed to study its relation to the variation of East Asian monsoon circulation in summer. The results are as follows:(1) ILSTD can be used to identify the strength of East Asian summer monsoon. The years of strong(weak) ILSTD Dare associated with stronger(weaker) summer monsoon circulation. (2) The index also can reflect summer rainfall anomaly over eastern China. In the years of strong ILSTD, rainfall belt is mainly located over northern China, and obvious drought emerges in Huaihe River valleys and the mid-lower reaches of Yangtze River. However, during the weak years, summer monsoon is weak and summer rainf all belt is located over the mid-lower reaches of Yangtze River. The circulation fields of strong (weak) index years can explain the features of rainfall anomaly. (3) Besides, the ILSTD has obviously QBO and quasi-4-year and interdecadal oscillations.