Motion Vector Re-estimation for Trans-coding Using Kalman Filter Recently, many visual devices are applied to mobile phone, PDA or other products. These devices can receive real-time videos from wireless network or internet. In order to watch video programs, the video frame sizes receiving from various devices must be properly changed. Although many fast downscaling algorithms have been proposed, such as SA, AWA and MDN, their performances do not achieve high visual quality. In this paper, we propose a method to address this problem. Considering the correlation of motion characteristics of neighbor blocks in temporal and spatial domain, the method applies embedded Kalman filter to traditional fast downscaling algorithms. The experimental results indicate that the performance of the proposed method for fast downscaling videos provides an effective improvement.