The meteorologically adjusted long term variations of the surface ozone concentrations over 6 major cities in Korea were determined by using the low-pass Kolmogorov and Zurbenko (KZ)-filter and multiple linear regressions. In addition, by using the OZone Isopleth Plotting package for Research-oriented version (OZIPR), the variation of ozone induced by the change of precursor emissions was separated from the long term variation of ozone over Seoul, thus the relative contribution of the background ozone concentration was deduced. Although both the effects of precursor emission and background level change have increased the surface ozone concentration from 2001 to 2008, it is inferred that the background effect is about 2.5 times higher than the precursor emissions change on the surface ozone concentration over Seoul. The relative importance of the background effect might be increasing as the emissions in Northeast Asia increase, while the effect of precursor emission could be reduced by continuous implementation of emission control strategies in Seoul.