Abstract: Methods for fast mode decision of variable size block coding referring to spatial and temporal correlations between a current encoding motion block and at least one reference motion block to decide a best mode for encoding the current encoding motion block. The at least one reference motion block includes at least one neighboring motion block of the current motion block and/or a previous motion block that is located in a previous image frame at a position corresponding to that of the current encoding motion block in a current image frame. At least one block size mode is obtained from the at least one reference motion block. The methods further check the reliability of the at least one block size mode before using the at least one block size to encode the current motion block.