Abstract: There are two parts in this paper. In the first part, the influences of the mountain range on the instability of barotropic disturbance are studied. The necessary condition for the occurrence of unstable disturbance is: (d2u/dy2)-β-fA(dh/dy)=0,where u is the basic current and h is the height of the mountain. Furthermore, the growth- rate of amplitude and the velocity of displacement of disturbance are also estimated roughly. In the second part, a further study of the same fopic in the baroclinic atmosphere is made. We found that, in the baroclinic atmosphere, the effect of vertical motion produced by northern and southern slopes of large mountain is more important.