Abstract: The regional and seasonal variations of surface O₃ over East Asia from 2002 to 2007 and the reason that the maximum of the O₃ monthly mean values occurred at different time at different sites are analysed by using the data of Acid Deposition Monitoring Network in East Asia,NCEP/NCAR,NOAA, and Global Precipiation Climatology Project. The results showed that the highest value occurred in spring and the lowest occurred in summer over East Asia and most midlatitude clean background areas in the Northern Hemisphere. Because of the East Asian monsoon advancement process the time when the monthly mean value in spring reaches the maximum are slightly different at different sites. AT Ogasawara and Hedo,they reached the highest values in March, while at Rishiri, Happo, Oki, and Cheju,they reached the highest values in April or May. This is mainly because of the push of East Asia summer monsoon. The O₃ concentration is higher than the climate mean in the spring of 2007. This is mainly because of the delay of subtropical high establishing and the block of the pushing. In 2007, the O₃ concentration reached the maximum in May at Cheju. The O₃ concentration in the spring 2004 at Hedo was significantly lower than average because of the earlier establishing of East Asia summer monsoon.