Abstract: A novel fast variable block size selection method for an inter-MB in a video-downsizing transcoder for H.264 is proposed,which uses the spatial homogeneity based on moving edge detection and temporal stationary characteristics by measuring activity magnitude.Based on the homogeneity and stationary of macro-blocks,only a few inter-modes are operated with the RDO process.In addition,adaptive motion vector re-construction and refinement scheme are proposed based on region of interest (ROI),besides of using variance and DCT coefficient for the definition of diversity and importance,respectively.Experimental results show that the new method is more robust and achieves good performance in both quality and complexity.