A deblocking filter apparatus (120, 220, 600) for use in an image encoder (100,1300,1400) and/or an image decoder (200,1300,1400), for deblocking a block edge (403, 504, 903) between a first image block (401, 501) and a second image block (402, 502), wherein the first image block (401, 501) has a block size being M*N or N*M, and M and N represent the width and height of the first image block respectively, or N and M represent the width and height of the first image block respectively; wherein the second image block (402, 502) has a block size being L*T or T*L, and L and T represent the width and height of the second image block respectively, or T and L represent the width and height of the second image block respectively, and wherein the deblocking filter apparatus is configured to perform the following steps, when one of the first image block and the second image block includes sub-blocks (4021, 4022; 5021, 5022) and the one image block including sub-blocks is the second image block (402, 502), and only when N and T are even integers 2 n larger than 16, and n is a positive integer: modify values of MA samples of the second image block (402, 502) as first output values, wherein the MA samples are in a line perpendicular to and adjacent to the block edge (403, 504, 903), MA is a positive integer equal to 3; and modify values of MB samples of the first image block (401, 501) as second output values, wherein the MB samples are in a line perpendicular to and adjacent to the block edge (403, 504, 903), MB is a positive integer equal to 7. wherein the first image block (401, 501) has a block size being M*N or N*M, and M and N represent the width and height of the first image block respectively, or N and M represent the width and height of the first image block respectively; wherein the second image block (402, 502) has a block size being L*T or T*L, and L and T represent the width and height of the second image block respectively, or T and L represent the width and height of the second image block respectively, and wherein the deblocking filter apparatus is configured to perform the following steps, when one of the first image block and the second image block includes sub-blocks (4021, 4022; 5021, 5022) and the one image block including sub-blocks is the second image block (402, 502), and only when N and T are even integers 2 n larger than 16, and n is a positive integer: modify values of MA samples of the second image block (402, 502) as first output values, wherein the MA samples are in a line perpendicular to and adjacent to the block edge (403, 504, 903), MA is a positive integer equal to 3; and modify values of MB samples of the first image block (401, 501) as second output values, wherein the MB samples are in a line perpendicular to and adjacent to the block edge (403, 504, 903), MB is a positive integer equal to 7. modify values of MA samples of the second image block (402, 502) as first output values, wherein the MA samples are in a line perpendicular to and adjacent to the block edge (403, 504, 903), MA is a positive integer equal to 3; and modify values of MB samples of the first image block (401, 501) as second output values, wherein the MB samples are in a line perpendicular to and adjacent to the block edge (403, 504, 903), MB is a positive integer equal to 7.