Abstract: It is convenient to use o-coordinate to discuss the dynamic effect of orography and the flow in Ekman boundary layer. Because the lower boundary condition in the coordinate is quite simple. In this paper, the theory of mixing length is generalized to the coordinate. Then the governing equations, describing tht motion in the boundary layer over the mountain are derived. The features of flow in the boundary layer, especially the effects of Ekman pumping are discussed in details. It is pointed out that there are three factors which influence the vertical motion at the top of the boundary layer. The first is vorticity distribution in the boundary layer. This is related with the divergence of air flow caused by the friction. The second is the climbing up or down of the air flow over the mountain slopes. The third is the geostrophic wind which flows along the mountain. This is due to the climbing of air flow components which deviate from the geotrophic wind, caused by the effect of friction.