A motion estimation method for a video processor includes steps of: calculating frame difference between a current input frame and a previous input frame; comparing the frame difference and a threshold to generate a comparison result; if the comparison result indicates the frame difference less than the threshold, determining whether a first block of the current input frame is a mixed block according to a block difference that indicates a difference between the first block and a block of the previous input frame at the same position; generating at least one temporal motion vector candidate for the first block; modifying the temporal motion vector candidate according to a current position difference value of video part in the current input frame when the first block is determined as the mixed block; and determining a motion vector for the first block from multiple motion vector candidates including the temporal motion vector candidate.