Abstract: The characteristics and causes of the East Asian summer monsoon (EASM) interdecadal variability during the Medieval Warm Period (MWP, 1001—1100 ) and Present Warm Period (PWP, 1901—2000) are examined by using results of all-forcing and control experiments over the past 1500 years of the Community Earth System Model version 1.0 (CESM1). Results show that the interdecadal variability of the EASM is characterized by a north-south dipole rainfall pattern in eastern China and the significant interdecadal periods of the EASM are around 10 and 20 years during both periods. These changes are mainly attributed to the internal variability, which is preliminarily dominated by the Pacific Decadal Oscillation. The positive phase of PDO induces warming (cooling) over the tropical western Pacific (East Asia continent), featuring a "colder land-warmer ocean" pattern, which favors a weak EASM. Besides, the meridionally arranged anomalous low-level anticyclone (cyclone) occurs over the northwestern Pacific (Sea of Japan) during MWP. These are dynamically in accordance with the sea level pressure, thus resulting in increased (reduced) precipitation over the southern China (northern China).