Abstract: Circulation differences and a possible mechanism for the establishment of the South Asia high (SAH) over the Indo-China peninsula (ICP) from April to May in early/delayed-establishment years are investigated using NCEP/NCAR daily reanalysis and outgoing longwave radiation (OLR) data from 1979 to 2008. It is found that the circulation pattern in early-establishment years is significantly different from that in delayed years in the upper and lower troposphere. In the upper troposphere, the center of the anticyclone to the east of the Philippines moves fast in the early years before the SAH establishment. Conversely, no closed anticyclone exists over the western Pacific in delayed years. The area of anticyclone over the ICP is wide (narrow) in the early (delayed) years after establishment, and the duration required for establishment of the SAH is relatively long (short) in the early (delayed)-establishment years. In the lower troposphere, both the wind and sea level pressure fields exhibit a reverse distribution. The intensity of the Walker circulation in early-establishment years is also different to that in delayed years. The time required for the establishment of the center of the anticyclone over the ICP is closely associated with convection over this region, and the earlier (later) convective activity corresponds to earlier (later) establishment of the center. Moreover, the convective activity occurs slightly earlier than the establishment of the center. The westward velocity and intensity of the anticyclonic center to the east of the Philippines are affected by vertical motion and atmospheric diabatic heating. After this anticyclonic center moves to 130°-145°E, vertical changes in diabatic heating play a major role in the maintenance of the center. The atmospheric thermal conditions and vertical motion over the ICP to the east of the Philippines are affected by the intensity of the Walker circulation, which is in turn determined by the distribution of anomalies in tropical Pacific sea surface temperature in the previous winter and spring. Such conditions can induce circulation differences during the establishment of the SAH.