Abstract: The present paper has studied the dynamic structures of the Siberian high during it's build-up. It has been found that at the initial stage of formation of the Siberian high, the positive vorticity dominates the region of the high in the troposphere. There is convergence at low-level and high-level, respectively, with divergence in between. Correspondingly, there is the upward motion in the layer below 700 hPa and downward motion above 700 hPa. At the mature stage of the high, the significant negative vorticity and strong divergent airflows are observed in the lower and middle troposphere while there are the positive vorticity and convergent airflows in the upper troposphere. The downward motion occurs in the whole layer of the troposphere. This fact shows that the strong mass convergence in the middle and upper troposphere is a major factor leading to the development of the Siberian high. The diagnostic analysis made by the use of the vorticity equation further indicates that the generation and intensificanon of the negative vorticity in the high troposphere mainly result from the effects of the advection of the negative vorticity in the middle and upper troposprere, whereas at low-level the divergence term(divergence) is the major factor for the generation of the negative vortieity. On the other hand, the heat budget of the Siberian high has revealed a deep layer of cooling, thus inducing the downward motion in the troposphere and then convergence at mid and high levels and divergence at low-level. This condition is favorable for the enhancement of the high. Therefore, the build-up of the Siberian high results from the combined effects of dynamic and thermal factors.