This study was conducted to examine the correlation between the South Asian High (SAH) shift and the summer (June-August) rainfall variability in Korea over the course of 45 years (1958-2002). It is found that the shift in the west-east direction is closely related to the summer rainfall variability in Korea. The atmospheric circulations in the east shift years showed that at 850 hPa, an intensification of the western North Pacific subtropical high allowed a sufficient amount of warm and humid air to be supplied to the lower atmosphere near Korea. On the other hand at 200 hPa, the SAH expanded toward the east, near the southern part of Korea, causing the intensification of the northwesterlies, and this allowed the supply of cold and dry air to the upper atmosphere near Korea. Meanwhile, in the latitudinal zone in which Korea is located, there were anomalous upward flows with warm and humid characteristics in the lower atmosphere and anomalous downward flows with cold and dry characteristics in the upper atmosphere. As such, the cold and warm air masses present in the upper and lower atmosphere near Korea led to thermal instability, and this was analyzed to be a major factor that could potentially increase the amount rainfall in Korea in summer in the east shift years. Moreover, a warm SST anomaly was formed in the mid-latitude zones of East Asia, and it heated the lower troposphere, thereby intensifying the thermal instability even further. Furthermore, TCs frequently had an impact on Korea in the east shift years, and were another factor that caused the increased rainfall in summer.