Motion estimation is a key technique in MPEG and H.263 encoder, due to its significant impact on the bit rate and the output quality of the encoded sequence. The full search algorithm (FS), which is considered to be the optimal, is computational intensive, and traditional fast algorithms still need to be improved in both computational complexity and matching accuracy. This paper proposes a novel algorithm, DSWSS, which is based on dynamic search window and spiral search. It performs as accurately as full search algorithm, but needs much less computation.