A group of numerical simulations has been conducted in order to investigate the effects of the Tibetan Plateau on the mean circulation in East Asia in the transitional season.The results show that the relative importance of the dynamic and the thermal effects of the plateau is chaged dramatically from May to June. In May as in winter the dynamic effect of the plateau plays a determinative role in the mean general circulation. However, in June the dynamic effect is reduced, the thermal effect becomes superior to it.Only when the thermal effect is taken into account in the model, the main systems of the general circulation can be fairly pro duced.