1. A motion estimation method for determining a resultant motion vector, the motion estimation method comprising: using an initial motion vector for performing a first searching operation according to a first searching principle, so as to determine an intermediate motion vector by a video processor, wherein the first searching operation is performed when a variation of motion vectors surrounding a current block is more than a predefined threshold, the first operation being performed by: (1) determining a main direction; (2) searching in the main direction according to a central block to determine a block having a minimum cost function value; (3) using the block having the minimum cost function value as the central block; and (4) searching in a direction orthogonal to the main direction according to the central block obtained in step (3) to determine a block having the minimum cost function value; and using the intermediate motion vector for performing a second searching operation according to a second searching principle, so as to determine the resultant motion vector by the video processor.