Abstract: In the free atmosphere, the geostrophic wind is the basic state of the atmospheric motion. The so-called geostrophic momentum approximation has been well developed to study the dynamics of non-uniform geostropliic wind. However, in the boundary layer, the basic motion is not geostrophic but Ekman flow. Thus, the geostrophic momentum approximation has to make some changes to characterize the feature of boundary layer. The purpose of the present study is to suggest an Ekman momentum approximation, which is similar to geostrophic momentum approximation in the free atmosphere. The physical bases of Ekman momentum approximation are studied in the paper. The wind structure and vertical motion at the top of the boundary layer under the approximation are studied in detail.