The relationship between the east Asia subtropical westerly jet (EASWJ) in the upper troposphere and the south Asian high and the thermal mechanism are examined by using the monthly mean NCEP/NCAR reanalysis data. The results show that the EASWJ in the upper troposphere and the south Asian high both experience seasonal variation, which migrate northward and westward from winter to summer and southward and eastward from summer to winter, but the change in the intensity is reverse. The EASWJ center is located in the northern side of the south Asian high, and the westerly wind to the northern side of the anticyclone, corresponding in the south Asian high, is composed of the major part of the westerly jet. During the monsoon season, the south Asian high center migrates northward prior to EASWJ center. In general, the Iranian high corresponds to the western jet pattern, and the Tibetan high to the eastern jet pattern. The pressure gradient contrast between the western jet pattern and eastern jet pattern is responsible for the westerly jet pattern difference. The westerly jet pattern is determined by the location of the south Asian high. Moreover, there exists apparent difference in the thermal mechanism, due to the south Asian high following the high temperature center.