A method of inter prediction encoding of an image, the method including: searching for a first reference block in a reference picture by using a current block, and estimating a first motion vector in a first pel unit in regards to the first reference block; estimating a second motion vector by using pixels included in a pre-encoded area adjacent to the current block, and pixels adjacent to the first reference block, and determining a second reference block based on the second motion vector; and encoding the current block based on the first motion vector and the second reference block.