Currently, various video formats, such as QCIF, CIF, CCIR601 and HDTV, are widely used in the world. Since their resolution is different, the processing speed required is different for motion estimation. Hence we need to design the specific hardware architecture for each format. In this study, we propose a flexible motion estimator to meet the processing speed of all formats with a common architecture, wherein there are four searching algorithms built to satisfy the various processing-time required. For applying to low-power systems, the computational kernel employs four processing-elements in this chip. With timing mode control, the throughput rate of the proposed motion estimator can achieve from 3k to 180k blocks to meet different applications while this chip works on 50MHz. The total gate count is less than 5k and the power dissipation is no more than 0.1mW in the worst case. Hence the very low-power motion estimation is appropriate for portable systems.