The rate-distortion optimal mode decision as well as motion estimation adopted in H.264 brings a big challenge to realtime encoding and transcoding duo to the high computation complexity. In this paper, we propose a hierarchical motion description model to present the motion data of each macroblock (MB) from coarsely to finely. A preprocessing approach is developed to estimate the motion data for each MB at each quality level with regard to its reference quality, its adjacent MBs and the target bit-rate. The resulting motion data can be coded and stored as metadata in a media file or a stream. Moreover, we propose a method to readily extract the specific motion data from the model for each MB at given bit-rates. Experimental results have shown the effectiveness of our proposed motion description model in terms of coding efficiency as well as fast bit-rate adaptation in comparison with that of H.264.