Experimental results show that the block motion field of a real world image sequence is usually gentle, smooth, and varies slowly, as a result in a center-biased global minimum motion vector distribution instead of an uniform distribution. Based on this characteristic of the image sequence a new four-step search (4SS) algorithm with a center-biased checking point pattern for fast block motion estimation is proposed. A variable searching-step technique is employed in the proposed algorithm with a minimum of 2 searching steps and a maximum of 4. The total number of checking points is varied from 17 to 27. Simulation results show that, as compared to the well-known three-step search, the proposed 4SS produces smaller motion compensation errors with a smaller computational requirement. The 4SS also possesses hardware-oriented features regularity and simplicity.