A method and a device for approximating a DC coefficient of a first block of pixels of a first frame are proposed. The method comprises: calculating a luminance DC average value based on DC coefficients of first frame's macro-blocks without an approximation error; and determining the DC coefficient of the first block based on the DC coefficient of a second block, wherein the second block is a part of a second frame, which is a reference frame of the first frame, the second block overlapping with a reference block of the first block and having the closest DC coefficient to the luminance DC average value.