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We apply a global three-dimensional Goddard Earth Observing System (GEOS) chemical transport model (GEOS-Chem) driven by NASA/GEOS-4 assimilated meteorological fields to quantify the impacts of the East Asian summer monsoon (EASM) on interannual variations of summertime surface-layer O3 concentrations over China.With anthropogenic emissions fixed at year 2005 levels, model simulation for years 1986-2006 shows that the changes in meteorological parameters alone lead to interannual variations in surface-layer O3 concentrations by 2-5% over central eastern China, 1-3% in northwestern China, and 5-10% over the Tibetan Plateau as well as the border and coastal areas of South China, as the interannual variations are relative to the average O3 concentrations over the 21 years.Over 1986-2006, O3 concentration averaged over the whole China is found to correlate positively with the EASM index with a large correlation coefficient of +0.75, indicating that JJA O3 concentrations are lower (or higher) in weaker (or stronger) EASM years.Relative to JJA surface-layer O3 concentrations in the strongest EASM years (1990, 1994, 1997, 2002, and 2006), O3 levels in the weakest EASM years (1988, 1989, 1996, 1998, and 2003) are lower over almost whole China with a nation mean lower O3 concentration by 2.0 ppbv (or 4%).Regionally, the largest percentage differences in O3 concentration between the weakest and strongest EASM years are found to exceed 6% in northeastern China, southwestem China, and over the Tibetan Plateau.Sensitivity studies show that the difference in transboundary transport of O3 is the most dominant factor that leads to lower O3 concentrations in the weakest EASM years than in the strongest EASM years, which, together with the enhanced vertical convections in the weakest EASM years, explain about 80% of the differences in surface-layer O3 concentrations between the weakest and strongest EASM years.We also find that the changes in the EASM strength are as important as the changes in anthropogenic emissions over 1986-2006 in influencing JJA surface-layer O3 concentrations in China.