Abstract: The general situation of longitudinal circulations over the Pacific low-latitude region of the northern hemisphere during summer, exhibits that to the west of 150°E there is the monsoon circulation, and to the east of 170°W there is the obvious trade circulation. From 150°E to 170°W lies a region of joining or transition between the monsoon and trade circulation. The horizontal circulation systems between the monsoon and the trade are the tropical upper tropos-pheric trough(TUTT)in the upper and the Pacific subtropical high in the lower. The dominent region of TUTT would be the joining or transition zone betweenthe monsoon and trade circulations.In this paper the structure of TUTT was analysed emphatically. Based on the vertical and horizontal circulations over the Pacific area, three-dimensional air-flow picture was given.