The location change of the westerly jet core at upper troposphere in June and July is investigated by using the NCEP/NCAR reanalysis data. The results show that the location of the westerly jet core changes rapidly from 140¿E to 90¿E during 35th--39th pentads, which corresponds to the plum rain period over East Asia. The location change of the jet core is actually the relative intensity change of the different westerly jet centers. The meridional temperature contrast in the troposphere is associated with the rapid location change of the jet core. The diabatic heating changes are the primary factor determining the seasonal evolution of the westerly jet core over East Asia.