Disclosed is a motion vector smoothing method. The motion vector smoothing method includes acquiring motion vectors of at least one block of a current image by using a reference image, acquiring a candidate motion vector of a current block, based on a motion vector of the current block, and at least one motion vector of at least one block adjacent to the current block, determining a threshold value of the current block according to gradient values of pixels included in the current block, and determining the motion vector of the current block according to the threshold value.