Ventilation or water exchange between the subtropical and tropic Pacific is studied with a simple none-thermodynamic two-and-half layer model. The shallow upper layer is assumed to communicate with its lower counterpart by entrainment and detrainment at ratesW e andW d. Trenberth wind stress data (Trenberth Data Center

1995) are chosen to drive the upper moving ocean to obtain a stable state. The results show that the subtropical and tropic Pacific exchange water in two ways. One is the direct exchanging by which the surface water in the mid-latitude goes down and moves to the tropical region in the subsurface layer and upwells back to the Ekman layer and finally moves northward to complete its journey; and the other is the way of detour

by which water in the subtropical Pacific moves first to the western boundary region along geostrophic flow contours and goes down to the tropic region as southward boundary currents. These two ways contribute almost the same amount to the water exchange between the subtropical and tropic Pacific gyres although the spatial scale in the latter is ten times greater than that of the former. LPS's (Luyten et al.

1983) ventilation theory is reviewed to give a clear picture of how the water exchange in large scale has been studied and the numerical results in this paper are compared with both the conclusions of such analysis theory and some previous observation (tritium survey) reports.